In a recent commit I changed flatpak_dir_list_remote_refs() to make main
refs (xa.main-ref) visible for noenumerate remotes (xa.noenumerate),
meaning that an origin remote created for a flatpakref file can be used
to get information about the ref from the file even before it is
installed. This commit extends that work to include extensions of the
main ref, and updates the unit test to check for this. The reasoning is
that if GNOME Software wants to show the user information about, say,
VLC from a flatpakref file before installing it, Software would want to
also show the available plugins so the user can decide if they want to
install those as well. I haven't checked if Software actually does that;
I'm only focusing on making libflatpak do the right thing.
6de22f0e62