Maybe there are no Go files at all.
Maybe they are all excluded by build constraints.
Maybe there are only test Go files.
Be specific.

Fixes #17008.
Fixes parts of #20760.

Change-Id: If6ac82ba0ed437772e76e06763263747d3bc4f65
Reviewed-on: https://go-review.googlesource.com/46427
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>