cmd/go/internal/renameio: add a ReadFile function

ReadFile is a drop-in replacement for ioutil.ReadFile that works
around Windows filesystem flakiness under load.

A followup CL will replace uses of ioutil.ReadFile in cmd/go with this
function.

Updates #32188

Change-Id: I232ba893b132bdc84cd7b0edde436165a69e1aa8
Reviewed-on: https://go-review.googlesource.com/c/go/+/180219


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>