Generalize the code to fetch Packages/Sources updates

It's now easy to extend by adding a supplementary lib/foobar-releases.mk
file.

I verified that the set of downloaded files are identical with the
exception of a bug that got fixed in the process: the update-backports-%
rule used to download files for all sections (main, contrib, non-free)
but saved them all under the name corresponding to the main section.