Instead of having a global config option we scan a directory for
symlinks into the sideload repos. These come from
/var/lib/flatpak/sideload-repos and /run/flatpak/sideload-repos (for
default system installation).
This is much easier to update atomically, and the two different
options are useful for persistant (the first) or dynamic (the second)
usescase.
Fixes https://github.com/flatpak/flatpak/issues/3494
15c761d482