oci: Break out get_digest_subpath helper function
oci: Break out get_digest_subpath helper function

matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak