Blender Git Loki

LibOverride: Refactor: Switch more code to using Main.relations.

This potentially could fix some missed cases in dependency tagging (when
dealing with overrides hierarchies), since relying on tag in ID itself
is not a good idea to check whether an ID has been propcessed or not
(exterior code may have forced that tag on some IDs e.g., which would
prevent them from ever being processed properly).