Many releases ago we migrated
from ioutil.ReadDir, which returned []os.FileInfo,
to os.ReadDir, which returns []fs.DirEntry.
The latter is faster, but the former is expected by go/build.Context.
Convert fsys to use the new ReadDir signature.
This should make the go command faster when scanning
source trees, and it brings cmd/go up to date with the rest
of the tree.
Similarly, convert Walk to WalkDir.
Change-Id: I767a8548d7ca7cc3c05f2ff073d18070a4e8a0da
Reviewed-on: https://go-review.googlesource.com/c/go/+/628698
Auto-Submit: Russ Cox <rsc@golang.org> Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Bypass: Russ Cox <rsc@golang.org> Reviewed-by: Sam Thanawalla <samthanawalla@google.com>