This commit essentially ensures before writing .blend file that only
actualy locally used linked IDs are tagged as `LIB_TAG_EXTERN` (and
therefore get a reference stored in the .blend file).
Previously, a linked ID tagged as directly linked would never get back
to the indirectly linked status. Consequence was a lot of 'needless'
references to linked data in .blend files.
This commit also introduces a new flag for lib_query ID usage types,
`IDWALK_CB_DIRECT_WEAK_LINK`, used currently for base's Object
pointer, and for LayerCollection's Collection pointer.
NOTE: A side-effect of this patch is that even IDs explicitely linked by
the user won't be kept anymore when writing files, i.e. they will not be
there anymore after a file reload, if they are not actually used.
Overhead of new process is below 0.1% in whole file saving process in
a Heist production file e.g.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D16605
133dde41bb