T7556: Rework, refactor, test, add version suffix

• Stop adding per-package gitlab-ci.yml, since we no longer need them
• Disable commit signing it .git/config so that it’s more convenient to work with the repo after the script has run
• Add version suffix to newly imported package, with a possibility to opt out
• Replace checkout-tarball and import-tarballs with pristine-lfs — we don’t want to maintain two different implementations of the same thing
• Add a set of basic tests, involving dgit as an independent but strict consumer of Git trees