cmd/go: spot-check the explicit requirements of root module dependencies when...

cmd/go: spot-check the explicit requirements of root module dependencies when loading packages from them

For #36460

Change-Id: I725ef5445b2bac7af827fb38373e8cd6dbad2d09
Reviewed-on: https://go-review.googlesource.com/c/go/+/316249


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>