In some cases, it's safe to avoid loading additional packages
when computing overlays. However it's not always safe to do so.
Avoid some unnecessary loads when it's completely safe to do so.
Updates golang/go#32538
Change-Id: Ie12204735940a540c9b3f29742f8479bcab5f077
Reviewed-on: https://go-review.googlesource.com/c/tools/+/181917
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>