xerrors: redirect Is, As, Unwrap to errors package

They're available in the standard library as of Go 1.13.

Change-Id: I8ff4f011a0904ce444e0e8c9b0ce9488668ba851
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/610077
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>