x/tools/go/packages: incorrect intermediate test variant computed for broken 'main' import

[findleyr]

In #66109, we see gopls running into a latent go/packages bug.

Consider the following setup (taken from the gopls regression test for that issue):

-- go.mod --
module example.com/tools

go 1.22

-- tools_test.go --
//go:build tools

package tools

import (
	_ "example.com/tools/tool"
)

-- tool/tool.go --
package main

func main() {
}

In this case, gopackages -test -json -deps ./tools_test.go will report a package "command-line-arguments [command-line-arguments.test]" with the nonsensical dependency ID "example.com/tools/tool [command-line-arguments.test]". This only occurs if the dep is a main package itself. It looks like go/packages thinks it has to break a cycle to the non-existent command-line-arguments package.

CC @adonovan @matloob

[gopherbot]

Change https://go.dev/cl/569035 mentions this issue: gopls/internal/cache: prune broken edges to command-line-arguments pkgs

[gopherbot]

Change https://go.dev/cl/569437 mentions this issue: [gopls-release-branch.0.15] gopls/internal/cache: prune broken edges to command-line-arguments pkgs