cmd/go: reject module path beginning with slash

[rsc]

Re #28336, we should

1. Reject go mod init /absolute/path.
2. Refuse to load any go.mod that says module /absolute/path.

If we'd done either of those (we should do both), then the confusing build errors in #28336 would not have happened.

[tandr]

To me it seems that denying full path in mod init would be counterproductive for people willing (or have no choice but) to specify a full path because thats how they have their build system set up

I would not be surprised to see this in a bootstrap script

go mod init /bigdisk/buildmount/$USER/$REPO/some/path

[jayconrod]

For reference, cmd/go/internal/module.CheckPath is the function go get uses to validate module paths. It performs a number of checks, not just for leading slashes.

In tests, I think we're using module paths that don't satisfy this check (e.g., the module path is a single word without a .). It's likely other people are taking advantage of this as well, both in tests and in monorepos that are never fetched with go get.

[emilyselwood]

Trailing slashes should probably also be blocked. It works as long as you don't have sub packages. As soon as you try to import a sub package you get errors like
build github.com/satappscatapult/warmconsidering/: cannot find module for path github.com/satappscatapult/warmconsidering/message

which is mighty confusing

series of comands

go mod init github.com/satelliteapplicationscatapult/warmconsidering/
code .
# write some things and create a submodule
go build
build github.com/satappscatapult/warmconsidering/: cannot find module for path github.com/satappscatapult/warmconsidering/message

edit the go.mod file to remove the trailing slash from the build starts to work as expected.

[gopherbot]

Change https://golang.org/cl/182560 mentions this issue: cmd/go: validate path in mod init path

[seankhliao]

This is now rejected with go: malformed module path "/example": empty path element in both cases