Origin URLs generated for Fedora origins

The Fedora lister currently uses rpm://fedora/packages/$package_name as origin URLs, inspired by the Debian lister's deb://Debian/packages/$package_name and deb://Ubuntu/packages/$package_name URLs.

However, neither deb: and rpm: are standard which risks conflicting with future specs (or other non-standard uses), and in itself it's not great.

While it is (probably?) too late to change Debian origins, we need to make a decision before deploying the Fedora lister in production.

Update (16/08/2023): An on going discussion on that subject can be found on the swh-devel mailing list (part 1, part 2).

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Migrated from T4632 (view on Phabricator)