cmd/go: add per-package indexing for modules outside mod cache

cmd/go: add per-package indexing for modules outside mod cache

Packages outside the module cache including the standard library will be
indexed individually rather than as a whole module.

For #52876

Change-Id: I142dad6a790e9e8eb4dc6430a588fbfa86552e49
Reviewed-on: https://go-review.googlesource.com/c/go/+/413815
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>