Fix T55007: Support fluid simulation cache files

This also adds support for entire directory paths to be assets, in
addition to globs and numbered file sequences.

tests/test_pack_progress.py

@@ -15,7 +15,7 @@ class ThreadedProgressTest(unittest.TestCase):
 
         # Flushing an empty queue should be fast.
         before = time.time()
-        tscb.flush(timeout=1)
+        tscb.flush()
         duration = time.time() - before
         self.assertLess(duration, 1)