Expand mod_internal tests to cover vendoring, replacements, and failure
messages.

Packages beginning with "golang_org/" resolve to $GOROOT/src/vendor, and should
therefore not be visible within module code.

Fixes #23970.

Change-Id: I706e9c4a1d1e025883e84b897972678d0fa3f2bd
Reviewed-on: https://go-review.googlesource.com/125836
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>