cmd/go: trim paths from vendored packages with -trimpath

In CL 199821, we stopped setting the module directory for vendored
packages when -mod=vendor is used. This broke -trimpath, since we
replace the module directory with a string derived from the module
path and version. A comment in CL 202977 makes it clear that the
module directory should not be set.

With this change, -trimpath falls back to replacing the package
directory with the package path if the module directory is not set. We
also fall back to replacing the package directory if the module
version is not set to avoid adding a meaningless @ only for the main
module.

As a consequence of this change, file names in vendored packages will
not have module versions, so file names will be a little different
between -mod=mod and -mod=vendor.

Fixes #36566

Change-Id: I0e9cd76d36a2028a49d0b6697ea9a9b3140d7ff3
Reviewed-on: https://go-review.googlesource.com/c/go/+/214945


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>