cmd/go: use index to match packages in dependency modules
If we're trying to search in a module in the module cache, instead
iterate over the packages in the index.
Change-Id: Ia94cbe6e9690110c28b93dbb33810680e3010381
Reviewed-on: https://go-review.googlesource.com/c/go/+/403756 Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Peter Weinberger <pjw@google.com>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>