bring back deb822 sources.list entries as .sources

bring back deb822 sources.list entries as .sources

Having two different formats in the same file is very dirty and causes
external tools to fail hard trying to parse them. It is probably not a
good idea for them to parse them in the first place, but they do and we
shouldn't break them if there is a better way.

So we solve this issue for now by giving our deb822 format a new
filename extension ".sources" which unsupporting applications are likely
to ignore an can begin gradually moving forward rather than waiting for
the unknown applications to catch up.

Currently and for the forseeable future apt is going to support both
with the same feature set as documented in the manpage, with the
longtime plan of adopting the 'new' format as default, but that is a
long way to go and might get going more from having an easier time
setting options than from us pushing it explicitely.