This method allows retrieving the owners listed in a file, as well as those
transitively listed in files included by it. This change also adds a new test to
cover this method, which is just a wrapper for the existing
_read_just_the_owners() internal method.

Bug: None
Change-Id: Iee956f115d3846acf0ee0806451807b0aa96d2f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1773904Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>