Add support for deb822 sources format

The goal of this MR is to add deb822 sources support to live-build without removing the existing support for the one-line sources format. Trixie and newer versions of Debian will automatically use deb822, Bookworm and older will continue to use the one-line format.

This is still a rough patch - I'm not 100% sure I've found all of the spots that need changed in order to implement this fully, there are some TODOs left in the test code (which I haven't even tried to run), and while I have gotten a successful ISO build with this, it has both a one-line format sources.list and a deb822 format sources.list.d/debian.sources for reasons I do not understand. Hopefully I'll have this more polished tomorrow or later within the week.

Implements (or, at least, will implement) https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1110886.