all: update to use os.ReadDir where appropriate

all: update to use os.ReadDir where appropriate

os.ReadDir is a replacement for ioutil.ReadDir that returns
a slice of fs.DirEntry instead of fs.FileInfo, meaning it is the
more efficient form.

This CL updates call sites throughout the Go source tree
wherever possible. As usual, code built using the Go 1.4
bootstrap toolchain is not included. There is also a use in
go/build that appears in the public API and can't be changed,
at least not without additional changes.

Fixes #42026.

Change-Id: Icfc9dd52c6045020f6830e22c72128499462d561
Reviewed-on: https://go-review.googlesource.com/c/go/+/266366
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>