cmd/go: forbid use of one module with two different paths

cmd/go: forbid use of one module with two different paths

If a single module is imported via two different paths (e.g., as itself and as a
replacement for something else), some users may be surprised if the two paths
do not share the same package-level state. Others may be surprised if the two
paths do share state.

Punt on the question for now by rejecting that condition explicitly.

Fixes #26607.

Change-Id: I15c3889f61f8dd4ba5e5c48ca33ad63aeecac04e
Reviewed-on: https://go-review.googlesource.com/126156
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>