proposal: cmd/go: add repo info to go list -m -json

[psantos1113]

When executing the command go list -m -u -json all having the root repository path and sub-module directory information would be useful in determining where exactly a module came from.
This would help determining where it was downloaded from and what tag was used to get the correct version of the module to debug any issues related to the module.
In the go module documentation there is the following example:

<meta name="go-import" content="golang.org/x/mod git https://go.googlesource.com/mod"

If the module imports golang.org/x/mod you would think that it is downloading the module from https://golang.org exactly.
Based on the result of the go-get query in the example, the repository where the module is downloaded from is https://go.googlesource.com/mod
This illustrates that the repository URL could be different.

Since go already calculates the repository root path and the submodule directory to obtain modules internally within go, I am proposing two additional fields to be added to the module data.
"RepositoryRootPath": "",
"ModuleSubdirectory": ""

Always show the RepositoryRootPath for all modules when listing them.
If the module is located in a subdirectory of the root repository, then add the ModuleSubdirectory to the output of the module.

What version of Go are you using (go version)?

$ go version
go version go1.15.5 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/user-1/Library/Caches/go-build"
GOENV="/Users/user-1/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/user-1/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/user-1/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/user-1/go/azure-autorest-test/src/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/s7/pngnf4m90m3gxp02xxpm5wkh0000gn/T/go-build227528808=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I created a sample module that imports modules that are in a subdirectory of a root repository.

main.go

package main

import (
    "fmt"
    "github.com/Azure/go-autorest/autorest"
    "github.com/Azure/go-autorest/autorest/azure"
    "github.com/Azure/go-autorest/autorest/validation"
    "github.com/Azure/go-autorest/autorest/to"
)

func main() {
    fmt.Println("Hello repository with subdirectory modules test!");
}

go.mod

module example.com/submodule-test

go 1.15

require (
	github.com/Azure/go-autorest/autorest v0.11.11
	github.com/Azure/go-autorest/autorest/to v0.4.0
	github.com/Azure/go-autorest/autorest/validation v0.3.0
)

What did you expect to see?

go list -m -u -json all Output:

{
	"Path": "example.com/submodule-test",
	"Main": true,
	"Dir": "/Users/user-1/go/submodule-test/src",
	"GoMod": "/Users/user-1/go/submodule-test/src/go.mod",
	"GoVersion": "1.15"
}
{
	"Path": "github.com/Azure/go-autorest",
        "RepositoryRootPath": "github.com/Azure/go-autorest",
	"Version": "v14.2.0+incompatible",
	"Time": "2020-06-23T16:10:07Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest@v14.2.0+incompatible",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/@v/v14.2.0+incompatible.mod"
}
{
	"Path": "github.com/Azure/go-autorest/autorest",
        "RepositoryRootPath": "github.com/Azure/go-autorest",
        "ModuleSubdirectory": "autorest",
	"Version": "v0.11.11",
	"Time": "2020-11-09T22:22:39Z",
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest@v0.11.11",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/@v/v0.11.11.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/autorest/adal",
        "RepositoryRootPath": "github.com/Azure/go-autorest",
        "ModuleSubdirectory": "autorest/adal",
	"Version": "v0.9.5",
	"Time": "2020-10-06T19:19:20Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest/adal@v0.9.5",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/adal/@v/v0.9.5.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/autorest/date",
        "RepositoryRootPath": "github.com/Azure/go-autorest",
        "ModuleSubdirectory": "autorest/date",
	"Version": "v0.3.0",
	"Time": "2020-06-23T18:36:26Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest/date@v0.3.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/date/@v/v0.3.0.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/autorest/mocks",
        "RepositoryRootPath": "github.com/Azure/go-autorest",
        "ModuleSubdirectory": "autorest/mocks",
	"Version": "v0.4.1",
	"Time": "2020-08-13T18:36:39Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest/mocks@v0.4.1",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/mocks/@v/v0.4.1.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/autorest/to",
        "RepositoryRootPath": "github.com/Azure/go-autorest",
        "ModuleSubdirectory": "autorest/to",
	"Version": "v0.4.0",
	"Time": "2020-06-23T18:36:26Z",
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest/to@v0.4.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/to/@v/v0.4.0.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/autorest/validation",
        "RepositoryRootPath": "github.com/Azure/go-autorest",
        "ModuleSubdirectory": "autorest/validation",
	"Version": "v0.3.0",
	"Time": "2020-06-23T18:36:26Z",
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest/validation@v0.3.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/validation/@v/v0.3.0.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/logger",
        "RepositoryRootPath": "github.com/Azure/go-autorest",
        "ModuleSubdirectory": "logger",
	"Version": "v0.2.0",
	"Time": "2020-06-23T18:36:26Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/logger@v0.2.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/logger/@v/v0.2.0.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/tracing",
        "RepositoryRootPath": "github.com/Azure/go-autorest",
        "ModuleSubdirectory": "tracing",
	"Version": "v0.6.0",
	"Time": "2020-06-23T18:36:26Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/tracing@v0.6.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/tracing/@v/v0.6.0.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/davecgh/go-spew",
        "RepositoryRootPath": "github.com/davecgh/go-spew",
	"Version": "v1.1.0",
	"Time": "2016-10-29T20:57:26Z",
	"Update": {
		"Path": "github.com/davecgh/go-spew",
		"Version": "v1.1.1",
		"Time": "2018-02-21T23:26:28Z"
	},
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/davecgh/go-spew@v1.1.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/davecgh/go-spew/@v/v1.1.0.mod"
}
{
	"Path": "github.com/form3tech-oss/jwt-go",
        "RepositoryRootPath": "github.com/form3tech-oss/jwt-go",
	"Version": "v3.2.2+incompatible",
	"Time": "2020-09-15T13:53:29Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/form3tech-oss/jwt-go@v3.2.2+incompatible",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/form3tech-oss/jwt-go/@v/v3.2.2+incompatible.mod"
}
{
	"Path": "github.com/pmezard/go-difflib",
    "RepositoryRootPath": "github.com/pmezard/go-difflib",
	"Version": "v1.0.0",
	"Time": "2016-01-10T10:55:54Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/pmezard/go-difflib@v1.0.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/pmezard/go-difflib/@v/v1.0.0.mod"
}
{
	"Path": "github.com/stretchr/objx",
        "RepositoryRootPath": "github.com/stretchr/objx",
	"Version": "v0.1.0",
	"Time": "2018-01-06T01:13:53Z",
	"Update": {
		"Path": "github.com/stretchr/objx",
		"Version": "v0.3.0",
		"Time": "2019-04-15T11:18:23Z"
	},
	"Indirect": true,
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/stretchr/objx/@v/v0.1.0.mod"
}
{
	"Path": "github.com/stretchr/testify",
        "RepositoryRootPath": "github.com/stretchr/testify",
	"Version": "v1.3.0",
	"Time": "2019-01-03T19:31:39Z",
	"Update": {
		"Path": "github.com/stretchr/testify",
		"Version": "v1.6.1",
		"Time": "2020-06-05T10:48:45Z"
	},
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/stretchr/testify@v1.3.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/stretchr/testify/@v/v1.3.0.mod"
}
{
	"Path": "golang.org/x/crypto",
        "RepositoryRootPath": "go.googlesource.com/crypto",
	"Version": "v0.0.0-20201002170205-7f63de1d35b0",
	"Time": "2020-10-02T17:02:05Z",
	"Update": {
		"Path": "golang.org/x/crypto",
		"Version": "v0.0.0-20201117144127-c1f2f97bffc9",
		"Time": "2020-11-17T14:41:27Z"
	},
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/golang.org/x/crypto@v0.0.0-20201002170205-7f63de1d35b0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/golang.org/x/crypto/@v/v0.0.0-20201002170205-7f63de1d35b0.mod",
	"GoVersion": "1.11"
}
{
	"Path": "golang.org/x/net",
        "RepositoryRootPath": "go.googlesource.com/net",
	"Version": "v0.0.0-20190404232315-eb5bcb51f2a3",
	"Time": "2019-04-04T23:23:15Z",
	"Update": {
		"Path": "golang.org/x/net",
		"Version": "v0.0.0-20201110031124-69a78807bb2b",
		"Time": "2020-11-10T03:11:24Z"
	},
	"Indirect": true,
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/golang.org/x/net/@v/v0.0.0-20190404232315-eb5bcb51f2a3.mod"
}
{
	"Path": "golang.org/x/sys",
        "RepositoryRootPath": "go.googlesource.com/sys",
	"Version": "v0.0.0-20190412213103-97732733099d",
	"Time": "2019-04-12T21:31:03Z",
	"Update": {
		"Path": "golang.org/x/sys",
		"Version": "v0.0.0-20201119102817-f84b799fce68",
		"Time": "2020-11-19T10:28:17Z"
	},
	"Indirect": true,
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/golang.org/x/sys/@v/v0.0.0-20190412213103-97732733099d.mod",
	"GoVersion": "1.12"
}
{
	"Path": "golang.org/x/text",
        "RepositoryRootPath": "go.googlesource.com/text",
	"Version": "v0.3.0",
	"Time": "2017-12-14T13:08:43Z",
	"Update": {
		"Path": "golang.org/x/text",
		"Version": "v0.3.4",
		"Time": "2020-10-21T14:53:29Z"
	},
	"Indirect": true,
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/golang.org/x/text/@v/v0.3.0.mod"
}

What did you see instead?

go list -m -u -json all

go list -m -u -json all Output

{
	"Path": "example.com/azure-test",
	"Main": true,
	"Dir": "/Users/user-1/go/azure-autorest-test/src",
	"GoMod": "/Users/user-1/go/azure-autorest-test/src/go.mod",
	"GoVersion": "1.15"
}
{
	"Path": "github.com/Azure/go-autorest",
	"Version": "v14.2.0+incompatible",
	"Time": "2020-06-23T16:10:07Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest@v14.2.0+incompatible",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/@v/v14.2.0+incompatible.mod"
}
{
	"Path": "github.com/Azure/go-autorest/autorest",
	"Version": "v0.11.11",
	"Time": "2020-11-09T22:22:39Z",
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest@v0.11.11",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/@v/v0.11.11.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/autorest/adal",
	"Version": "v0.9.5",
	"Time": "2020-10-06T19:19:20Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest/adal@v0.9.5",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/adal/@v/v0.9.5.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/autorest/date",
	"Version": "v0.3.0",
	"Time": "2020-06-23T18:36:26Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest/date@v0.3.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/date/@v/v0.3.0.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/autorest/mocks",
	"Version": "v0.4.1",
	"Time": "2020-08-13T18:36:39Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest/mocks@v0.4.1",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/mocks/@v/v0.4.1.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/autorest/to",
	"Version": "v0.4.0",
	"Time": "2020-06-23T18:36:26Z",
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest/to@v0.4.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/to/@v/v0.4.0.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/autorest/validation",
	"Version": "v0.3.0",
	"Time": "2020-06-23T18:36:26Z",
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/autorest/validation@v0.3.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/autorest/validation/@v/v0.3.0.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/logger",
	"Version": "v0.2.0",
	"Time": "2020-06-23T18:36:26Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/logger@v0.2.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/logger/@v/v0.2.0.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/Azure/go-autorest/tracing",
	"Version": "v0.6.0",
	"Time": "2020-06-23T18:36:26Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/!azure/go-autorest/tracing@v0.6.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/!azure/go-autorest/tracing/@v/v0.6.0.mod",
	"GoVersion": "1.12"
}
{
	"Path": "github.com/davecgh/go-spew",
	"Version": "v1.1.0",
	"Time": "2016-10-29T20:57:26Z",
	"Update": {
		"Path": "github.com/davecgh/go-spew",
		"Version": "v1.1.1",
		"Time": "2018-02-21T23:26:28Z"
	},
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/davecgh/go-spew@v1.1.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/davecgh/go-spew/@v/v1.1.0.mod"
}
{
	"Path": "github.com/form3tech-oss/jwt-go",
	"Version": "v3.2.2+incompatible",
	"Time": "2020-09-15T13:53:29Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/form3tech-oss/jwt-go@v3.2.2+incompatible",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/form3tech-oss/jwt-go/@v/v3.2.2+incompatible.mod"
}
{
	"Path": "github.com/pmezard/go-difflib",
	"Version": "v1.0.0",
	"Time": "2016-01-10T10:55:54Z",
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/pmezard/go-difflib@v1.0.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/pmezard/go-difflib/@v/v1.0.0.mod"
}
{
	"Path": "github.com/stretchr/objx",
	"Version": "v0.1.0",
	"Time": "2018-01-06T01:13:53Z",
	"Update": {
		"Path": "github.com/stretchr/objx",
		"Version": "v0.3.0",
		"Time": "2019-04-15T11:18:23Z"
	},
	"Indirect": true,
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/stretchr/objx/@v/v0.1.0.mod"
}
{
	"Path": "github.com/stretchr/testify",
	"Version": "v1.3.0",
	"Time": "2019-01-03T19:31:39Z",
	"Update": {
		"Path": "github.com/stretchr/testify",
		"Version": "v1.6.1",
		"Time": "2020-06-05T10:48:45Z"
	},
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/github.com/stretchr/testify@v1.3.0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/github.com/stretchr/testify/@v/v1.3.0.mod"
}
{
	"Path": "golang.org/x/crypto",
	"Version": "v0.0.0-20201002170205-7f63de1d35b0",
	"Time": "2020-10-02T17:02:05Z",
	"Update": {
		"Path": "golang.org/x/crypto",
		"Version": "v0.0.0-20201117144127-c1f2f97bffc9",
		"Time": "2020-11-17T14:41:27Z"
	},
	"Indirect": true,
	"Dir": "/Users/user-1/go/pkg/mod/golang.org/x/crypto@v0.0.0-20201002170205-7f63de1d35b0",
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/golang.org/x/crypto/@v/v0.0.0-20201002170205-7f63de1d35b0.mod",
	"GoVersion": "1.11"
}
{
	"Path": "golang.org/x/net",
	"Version": "v0.0.0-20190404232315-eb5bcb51f2a3",
	"Time": "2019-04-04T23:23:15Z",
	"Update": {
		"Path": "golang.org/x/net",
		"Version": "v0.0.0-20201110031124-69a78807bb2b",
		"Time": "2020-11-10T03:11:24Z"
	},
	"Indirect": true,
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/golang.org/x/net/@v/v0.0.0-20190404232315-eb5bcb51f2a3.mod"
}
{
	"Path": "golang.org/x/sys",
	"Version": "v0.0.0-20190412213103-97732733099d",
	"Time": "2019-04-12T21:31:03Z",
	"Update": {
		"Path": "golang.org/x/sys",
		"Version": "v0.0.0-20201119102817-f84b799fce68",
		"Time": "2020-11-19T10:28:17Z"
	},
	"Indirect": true,
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/golang.org/x/sys/@v/v0.0.0-20190412213103-97732733099d.mod",
	"GoVersion": "1.12"
}
{
	"Path": "golang.org/x/text",
	"Version": "v0.3.0",
	"Time": "2017-12-14T13:08:43Z",
	"Update": {
		"Path": "golang.org/x/text",
		"Version": "v0.3.4",
		"Time": "2020-10-21T14:53:29Z"
	},
	"Indirect": true,
	"GoMod": "/Users/user-1/go/pkg/mod/cache/download/golang.org/x/text/@v/v0.3.0.mod"
}

[seankhliao]

The go command may not have this information if it only communicates with a proxy. Additionally this information is not guaranteed to be stable over time since code host can change, or even available.

[mvdan]

If people really want this, there could be a (likely third party) tool to work it out from the output of go list -m. I've thought about it in the past, but not implemented it yet. Beware that, like @seankhliao says, it's a best-effort reconstruction since Go modules are decoupled from VCS servers/systems.

Nested or overlapping modules also make this reconstruction work much trickier than it might seem.

[rsc]

This doesn't seem viable to me. The default Go config is to use the proxy, which does not serve this information. Making go list -m -json do additional repo work would slow it down, to provide info that very few use cases need. As others noted, we have worked very hard to completely separate modules from the underlying repos.

Doing this in a separate package makes more sense. (It need not be a standard or golang.org/x package.)

[bcmills]

The .info files served by proxies could theoretically report this information (where they already record the original commit timestamp).

It would be somewhat analogous to the “downloaded on…” citations you sometimes see in research papers that cite websites: it would tell you where the proxy got its copy of the module from, but the module wouldn't necessarily still be available from that source.

[seankhliao]

I remember filing #44742 for having the info recorded in the .info files