Don't auto-install sources for extensions
Don't auto-install sources for extensions

There was a bug in the extension point matcher which made it
install `org.gnome.Totem.Videosite.YouTubeDl.Sources` (in addition to
`org.gnome.Totem.Videosite.YouTubeDl`) for the `org.gnome.Totem.Videosite`
extension.

We just need to make sure we only match the extension prefix if there
is a single element in the extension name following the extension
name (i.e. '.YouTubeDl', not '.YouTubeDl.Sources').

This fixes https://github.com/flatpak/flatpak/issues/3973

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak