Previous code was doing N collection syncs when importing N objects
(essentially quadratic complexity in terms of object count). New
code avoids all the intermediate syncs by using
BKE_layer_collection_resync_forbid and
BKE_layer_collection_resync_allow, and then does one
BKE_main_collection_sync + BKE_main_collection_sync_remap for the
whole operation. The things done on the importer objects that are
dependent on the sync happening (marking them selected) are done in a
separate loop after the sync.
Timings: importing Moana USD scene (480k objects) on Windows, VS2022
Release build, AMD Ryzen 5950X: 12344sec -> 10979sec (saves 22 minutes).
Reviewed By: Bastien Montagne
Differential Revision: https://developer.blender.org/D15215
230f72347a