cmd/go/internal/modcmd: error out if one module with two different paths

cmd/go/internal/modcmd: error out if one module with two different paths

If a single module is imported via two different paths, go mod tidy
should have reported this error instead of deferring it until go build.

Fixes #34650.

Change-Id: I9d09df1551b3e2083ed9f0bc77f2989073057717
Reviewed-on: https://go-review.googlesource.com/c/go/+/199598
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>