transaction: Add back support for file: uris in RuntimeRepo keys
transaction: Add back support for file: uris in RuntimeRepo keys

This used to work, and the gnome-software test suite relies on it,
so add it back. I believe it regressed in
 https://github.com/flatpak/flatpak/pull/2740

This fixes https://github.com/flatpak/flatpak/issues/2955

Closes: #2956
Approved by: alexlarsson

flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak