Looking for vendor directories is a better default.

Fixes #13772

Change-Id: Iabbaea71ccc67b72f14f1f412dc8ab70cb41996d
Reviewed-on: https://go-review.googlesource.com/18450Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>