Pull the scripts from the package-source-builder repo

Move here the scripts currently embedded in the
`apertis-package-source-builder/overlay/usr/bin/` folder of the
apertis-docker-images> repository so we collect them all in a single place:

* apertis-pkg-merge-updates
* apertis-pkg-merge-upstream-to-downstreams
* apertis-pkg-pull-updates
* ci-buildpackage
* ci-license-scan
* ci-obs-upload
* deb-git-version-gen
* osc-setup
* tally-revision

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>