Previously, if an OWNERS file included //foo/API_OWNERS, then the
code would get confused and think that it had already read and processed
//foo/OWNERS, transferring the contents of the former to the latter.
This was wrong. This change fixes the attribution and adds tests to make
sure we catch this in the future.

R=thakis@chromium.org
BUG=697156

Change-Id: I1f1b846cafac2ad6d792d2dccfce94911e9d15c3
Reviewed-on: https://chromium-review.googlesource.com/447962Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>