When the Asset Browser shows the "Current File" asset library, now it
also attempts to load an asset catalog definition file from location of
the current .blend file. This happens as follows:
* First, see if the file is inside of an asset library that is "mounted" in the
Preferences. Load the catalogs from there if so.
* Otherwise, if the file is saved, load the catalogs from the directory the
file is saved in.
* If the file is not saved, no catalogs will be loaded.
Unit tests are being worked on in D12689.
Creating catalogs from the "Current File" asset library still doesn't work, as
the asset catalog service doesn't construct an in-memory catalog definition
file in that case yet.
Differential Revision: https://developer.blender.org/D12675
adaf4f56e1