Blender Git Loki

Fix (unreported) critical mistake in Collection liblink code in readfile.c.

Prelimenary step to fix T77460.

Not sure how or when that thing was done, but since that call walks
around collections relationships, it's an utterly critical violation of
liblinking principles (code here should never, ever 'get outside' of its
own ID scope).

This was wroking so far only because code called through this function
(`BKE_collection_parent_relations_rebuild`) was only following parents
pointers (in `BKE_collection_find_cycle()`), which would be either valid
or non-existent.

But next commit is going to change that to also check collection's
objects instancing of other collections.