Implement handling of source tarballs for pkgbases

When we are providing GPL packages, we need to also provide their sources, as the license requires us to do so. For Arch Linux this is currently done by a script, that runs on a timer and builds source tarballs for any package in the pool which is (partly) licensed under any of the GPL licenses. Therefore we need to implement the handling of source tarballs in their respective repository. The settings module is already aware of source repository directories for each package repository.

Note: For license identification an overhaul of how licenses are currently being used is currently evaluated.