cmd/go: report original module path in error parsing replaced go.mod

MVS reports an error when a go.mod file declares a module path that
doesn't match the path it was required with. If the module is a
replacement, its declared path may be the original path (preferred) or
the replacement path.

This CL makes the reported error a little more clear: the "required as"
path should be the original required path, not the replacement path.

Fixes #38220

Change-Id: I08b50a100679a447c8803cca1d1b32bc115ec1b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/227097


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>