x/tools/gopls: find references from go.mod files

[findleyr]

A little feature I've always wanted: when looking at a go.mod flie, I want to jump to uses of a module.

For example, today I was investigating postgres usage in pkgsite, and I wanted to see where github.com/lib/pq was used. I can grep, but that's a different UX and loses some fidelity.

I think this follows the theme of "make more LSP operations work more of the time", and should be straightforward to implement given that we already support find references on package paths, and this would just be a union of those results.

@adonovan @pjweinb WDYT, worth doing?

[adonovan]

Assuming that the only use of a module is an import of one of its packages, then the simplest and most efficient implementation would be to scan the metadata for all imports and then scan the File.Imports specs. You wouldn't need an index at all.

[pjweinb]

It doesn't seem hard to do. What UI would you want? (presumably like show references, pointing to the import statements?)

[findleyr]

Yeah, I think the trigger should be the ordinary LSP references request, dispatching to the gopls/internal/mod package.

[findleyr]

One place where this is strictly better than grep: if I find uses of an indirect require statement, I'll find them in the module cache.