[Merge] ~juliank/livecd-rootfs:deb822-sources into livecd-rootfs:ubuntu/master

[Julian Andres Klode]

This is v3. The main change is that we now keep a transitional sources.list with a nice comment pointing users to sources.list.d. It probably ought to have a link to a post or something.

This change is surprisingly crucial: Previous iterations tried removing the sources.list file instead; but then it kept reappearing unexpectedly, despite there not being lb_chroot_archives calls in between.

I have verified this branch on building and installing the 'ubuntu' project, and I have also built and inspected the output for the 'ubuntu-cpc' project.
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https://code.launchpad.net/~juliank/livecd-rootfs/+git/livecd-rootfs/+merge/458366
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