cmd/go: add basic support for overlays

This CL adds basic support for listing packages with overlays.
The new cmd/go/internal/fs package adds an abstraction for communicating
with the file system that will open files according to their overlaid paths,
and provides functions to override those in the build context to open
overlaid files. There is also some support for executing builds on packages
with overlays. In cmd/go/internal/work.(*Builder).build, paths are mapped
to their overlaid paths before they are given as arguments to tools.

For #39958

Change-Id: I5ec0eb9ebbca303e2f1e7dbe22ec32613bc1fd17
Reviewed-on: https://go-review.googlesource.com/c/go/+/253747


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