go/build: add the ability to list packages

[josharian]

This arose in the context of CL 9001, which @mdempsky wrote:

Lastly, an alternative approach I was considering was looking into moving cmd/go's package list resolving into go/build (or an internal helper package). Then cmd/api and deps_test.go could simply do something like ctxt.List("std") without needing to run cmd/go.

I have wanted this myself. I have a couple of (private) tools that shell out to go list std to get a listing of the stdlib's packages. It'd be nice not to have to do that.

[fd0]

I'd really like to be able to list packages instead of having to exec go list.