cmd/godoc: add x/xerrors redirect
cmd/godoc: add x/xerrors redirect

Change-Id: I367caa5f8c90e53768b07427248ed2bcc83a58e3
Reviewed-on: https://go-review.googlesource.com/c/159739
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools