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>