Move cache_dir from site-packages to local folder (#5856)

Previously cache_dir was located under site-packages, which is managed
by blender and could be read-only. Now location defaults to eg.
~/.cache/bonsai or equivalent location on other platforms. See #5564

Note this adds a dependency on the platformdirs python module

src/bonsai/test/tool/test_debug.py

@@ -55,7 +55,7 @@ class TestLoadExpress(NewFile):
 
 class TestPurgeHdf5Cache(NewFile):
     def test_run(self):
-        cache_dir = Path(bpy.context.scene.BIMProperties.data_dir) / "cache"
+        cache_dir = Path(bpy.context.scene.BIMProperties.cache_dir)
         test_file = cache_dir / "test.h5"
         test_file.parent.mkdir(parents=True, exist_ok=True)
         test_file.touch()