This is really not elegant solution, but simplest for now.
Ideally, we'd check all IDs first and directly 'make local' (without need to copy)
those only indirectly used by other datablocks to be made local. Would also save us from the
need of the extra 'lib_local' parameter recently added to id_make_local.
Current code seems to work well enough though, and this is not high priority to fix imho.
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