flatpak: Implement gs_plugin_add_search_what_provides()
Let the Flatpak plugin search for "what provides" as well. It's the same search, due to the appstream search terms, and it gets more results to pick from.
Related GNOME/gnome-software#613 (closed) Closes GNOME/gnome-software#1210 (closed)
(cherry picked from commit 25269187)