packaging-updates: Ensure all available updates are reported

The current code was not reporting updates for destinations like debian/buster-updates unless the branch already existed.

Tweak the code to check all the sources even if only a subset of the matching branches exists, using a new base key in the source definition to group them.

For instance, if a repository has only the debian/buster-updates branch the base key points to debian/buster which in turn matches the debian/buster, debian/buster-security, and debian/buster-updates sources. The availability of updates will be checked for each of them.