CI: Adjust CI to only process changed packages in Merge Requests

Currently, the CI checks all packages in every case, producing CI errors on every MR if a single package on master has a problem, even if those MRs have nothing to do with the respective package.

This is not necessarily the desired behaviour and shall be replaced with a new mechanism that only scans the affected packages during a CI run for an MR, while still running across the whole overlay tree when running the CI on master.