appstream: Catch on Appstream data creation race

This will help in situations when the Appstream data caches are empty and the corresponding directories nonexistent. The race is that in case for example the PackageKit is started due to the gnome-software request, then before it downloads and installs the repositories' Appstream data, the gnome-software's appstream plugin is already running and initialized, ignoring any additions into these paths, thus even the PackageKit installs the Appstream data, the gnome-software does not know about it either until restarted or when there's a change in any of the existing directories it already listens for the changes at.

Rather than keeping a list of current and legacy paths on multiple places, this checks only for the "swcatalog" paths, considering them being "the source of the truth" these days.