Use Debian published upstream listings

[frozencemetery]

Debian packages indicate their upstreams in the watch file. It would probably be good to incorporate this information rather than duplicating the effort.

[pypingou]

anitya is distro agnostic and in fact there are already a few projects there linked to Debian on it: https://release-monitoring.org/distro/Debian/

[ralphbean]

@pypingou, I think @frozencemetery is suggesting that we could script something to pull in all of the pre-existing Debian watch files and use them to auto-populate anitya (which could be cool).

[pypingou]

@ralphbean agreed :)