Alternatively, we could expose an appropriate API in the main repo. This seems like the sort of thing that would be appropriate to expose in go/build.

(We could possibly also create a go/vcs package, but it's less clear to me that that is a good idea. Want to file a separate issue about go/vcs?)

By vcs package, I mean https://godoc.org/golang.org/x/tools/go/vcs.
cmd/go should vendor it and use it instead of maintain a copy in vcs.go.

Whether to add new API to go/build depends on how confident we are.
That's why I suggest adding API to x/tools/go/build first and see what
the community responds. We can always migrate it to go/build if we're
certain they are the correct design.