cmd/go/internal/fsys: move glob, walk code into new files
The Glob and Walk code does not depend on any of the fsys internals;
it simply uses ReadDir as an opaque abstraction.
Move it to separate files so that when working on the
actual overlay abstraction, it is out of sight, out of mind.
Change-Id: Ifa98feaaaafe5c1d8d8edce82de4fd0c78f599c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/628696
Auto-Submit: Russ Cox <rsc@golang.org> Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
TryBot-Bypass: Russ Cox <rsc@golang.org> Reviewed-by: Michael Matloob <matloob@golang.org>