As of golang/go#50827, gopls no longer supports building at 1.12, and so
usage of golang.org/x/xerrors can be replaced with the native support for
error wrapping introduced in Go 1.13.
Remove this usage as a step toward eliminating the xerrors dependency
from x/tools.
For golang/go#52442
Change-Id: Ibf459cc72402a30a6c2735dc620f76ed8a5e2525
Reviewed-on: https://go-review.googlesource.com/c/tools/+/401097
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>