cmd/go: unify trimpath logic for -mod=vendor, -mod=mod

If a package has a module with a version, the package's directory is
replaced with the module path and version, followed by the package's
path within the module.

This is a follow up to CL 214945. We no longer check whether the
module has a directory (with -mod=vendor, it does not).

Updates #36566

Change-Id: I5bc952b13bc7b4659f58ee555bd6c6a087eb7792
Reviewed-on: https://go-review.googlesource.com/c/go/+/215940


Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>