Improve handling of (in)direclty linked status for linked IDs.

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

tests/python/bl_blendfile_liblink.py

@@ -247,7 +247,7 @@ class TestBlendLibLinkIndirect(TestBlendLibLinkHelper):
 
         assert material.users == 2
         # Currently linked data which has no more local user never gets reset to indirectly linked status.
-        # ~ assert material.is_library_indirect == True
+        assert material.is_library_indirect == True
 
         bpy.ops.wm.open_mainfile(filepath=output_work_path, load_ui=False)
 
@@ -264,7 +264,7 @@ class TestBlendLibLinkIndirect(TestBlendLibLinkHelper):
         assert material.users == 2  # Fake user is not cleared when linking.
         # Currently even re-reading the .blend file will not properly reset tag for indirectly linked data,
         # if their reference was written in the .blend file.
-        # ~ assert material.is_library_indirect == True
+        assert material.is_library_indirect == True
 
         assert mesh.library is not None
         assert mesh.use_fake_user is False