cmd/go: support overlays for synthesized packages.

The main missing piece here was supporting Stat in the overlay
filesystem, in the parts of the package code that determines whether
an command line argument is a file on disk or a directory.  so this
change adds a Stat function to the fsys package. It's implemented the
same way as the already existing fsys.lstat function, but instead of
os.Lstat, it calls os.Stat on disk files.

Then, the change changes parts of the package code to use the overlay
Stat instead of the os package's Stat.

For #39958

Change-Id: I8e478ae386f05b48d7dd71bd7e47584f090623df
Reviewed-on: https://go-review.googlesource.com/c/go/+/262617


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>