This is a simple change to the command that should resolve problems like finding
vendored packages before their non-vendored siblings. By searching in breadth-first
order, we find the matching package lowest in the hierarchy, which is more likely
to be correct than the deeper one, such as a vendored package, that will be found
in a depth-first scan.

This may be sufficient to resolve the issue, and has the merit that it is very easy
to explain. I will leave the issue open for now in case my intuition is wrong.

Update #12423

Change-Id: Icf69e8beb1845277203fcb7d19ffb7cca9fa41f5
Reviewed-on: https://go-review.googlesource.com/17691Reviewed-by: Russ Cox <rsc@golang.org>