CI: Backport source package handling changes to 6.1

The main part of this is a backport of !1675 (merged), fixing orig tarball caching and lintian coverage of source packages.

In preparation for that, it also:

• Changes genorig.py etc. to put orig tarballs directly in .., matching what uscan would do
• Backports merge request !1663 (merged), supporting the use of suffixed source package names