Backport !1919 “flatpak: Lookup for an alternate app skipping the alternate_of app” to gnome-46
requested to merge pwithnall/gnome-software:backport-1919-flatpak-file-source-gnome-46 into gnome-46
The gs_plugin_flatpak_find_app_by_ref() simply returns the first match, but it's not correct when looking for the alternatives, thus skip the alternate_of app to really return another version of the app.
Also add both the found alternative and the alternate_of apps into the result for an app with a local file, thus they are always both available in the GUI.
Related to #2488 (closed)
Trivial backport of !1919 (merged) to gnome-46.