check-invariants: Do not complain about missing upstreams for dropped packages

If we already dropped a package from the development channel there is no point in complaining if its upstream got dropped from Debian, even if we still ship in stable releases.

If instead it's still in a development branch, show the branch affected.

This silences errors on bootchart2 which is still in v2020 while it got dropped from Debian Buster and we no longer ship it in v2021 and later.