cmd/go: add test to check for a potential workspace loading issue

cmd/go: add test to check for a potential workspace loading issue

This test checks that we load the same graph regardless of the path to
the requested module in the workspace.

We currently don't. This will be fixed in a future change that redoes
workspace mode's usage of the Requirements structure.

For #45713

Change-Id: Id02cbb60a38619d840dbf1e70173ce853c0c167a
Reviewed-on: https://go-review.googlesource.com/c/go/+/348649
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>