flatpak: Centralise remote integration in FlatpakBackend
- Ensure remotes are integrated as sources when an installation transaction adds them
- Make sure we have a source available as soon as we know a remote is present, so it can be used.
I'd suggest merging back into 5.24 as it fixes a rather important crash. For me it's not as reproducible as for reporters so testing would be very welcome. In exchange it also fixes the flatpaktest that had been broken for a while.