repo: opportunistically populate RemoteCache from go.mod

[jayconrod]

What type of PR is this?

Bug fix

What package or component does this PR mostly affect?

language/go
repo

What does this PR do? Why is it needed?

Opportunistically populates RemoteCache from go.mod if the Go extension is being used and a go.mod file is in the repository root. This doesn't override roots added from WORKSPACE (or other configuration file). It should allow static external dependency resolution to succeed more often in go_repository when other go_repository declarations are outside the main WORKSPACE file, for example, in another dependency's macro.

Which issues(s) does this PR fix?

Fixes #1385

Other notes for review

This unfortunately introduces more Go-specific code to //cmd/gazelle. I don't think there's a good way to do it without refactoring //repo into //language/go first, and that's a non-trivial amount of work.

[linzhp]

Have you thought about adding a directive or command line flag to fix-update to provide the path to go.mod? That way we don't need to guess the location.

[jayconrod]

Have you thought about adding a directive or command line flag to fix-update to provide the path to go.mod? That way we don't need to guess the location.

I think for situations where this feature is useful, the go.mod file should always be in the workspace root directory.

Namely, I want this to work in go_repository when the author of the main WORKSPACE file might not be aware they have Go dependencies at all (because they are hidden inside a dependency macro). At this point, go_repository should nearly always be working in module mode; no point in supporting GOPATH mode any longer. In module mode, a go.mod can only appear in the module's root directory. A go.mod file in a subdirectory marks the root of another module, so that whole subdirectory is excluded from the module zip file.

For the main workspace, I'd encourage folks to add # gazelle:repository_macro or # gazelle:repository directives to the main WORKSPACE file instead of customizing the go.mod location, since those already work well and are more general.