This makes code behind the `bpy.data.libraries.write()` API use the new `PartialWriteContext` class, instead of the old `BKE_blendfile_write_partial` API. NOTE: This also means that the `blendfile_io` tests using this python API are now using the new class. No behavioral changes are expected here. Pull Request: https://projects.blender.org/blender/blender/pulls/122118