dir: Don't waste time reading metadata for the wrong ref
dir: Don't waste time reading metadata for the wrong ref

We don't support extensions of extensions, as evidenced by add_related()
only being called once in flatpak_transaction_real_run(), so there's no
need to read the metadata file of an extension in
flatpak_dir_find_local_related(), only to find that it doesn't have any
extensions of its own.

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