Similar to `bpy.data.user_map`, it returns a mapping of IDs to all the
filepaths they use.
Fairly basic still, may need to be refined with more options to
control which filepaths are included etc.
Mainly intended to make handling of a production blendfile dependencies
more easy.
Also adds some basic testing of the new feature.
Pull Request: https://projects.blender.org/blender/blender/pulls/127252
These are based on data generated for liblink tests, so part of this
code is also refactored to move data generation into `bl_blendfile_utils.py`.
NOTE: Work done also to add tests to upcoming filepath mapping utils
(see !127252).
Pull Request: https://projects.blender.org/blender/blender/pulls/133110