Support building dependency packages
Dependency packages are packages that are handled only for Debian package building purposes. Their jobs are stored in a separate folder that doesn't clutter the main swh package views.
To do so, we duplicate the gbp-buildpackage and automatic-backport jobs so they can be created in a separate folder. This is just a matter of playing with yaml anchors. We also adapt the incoming-tag job to look for jobs in both places.
Test Plan
See jobs for mirakuru, everything seems happy
Migrated from D1104 (view on Phabricator)