Pack: always pack files, explode directories to a list of files

When an asset is represented as directory in Blender (for example fluid
simulation caches), that directory is traced and each file is considered
an asset.

This makes it considerably easier for Shaman clients, as they need to
compute the SHA256 checksum of each file. The logic to transform a
directory path to a list of the contained files is now in BAT itself.

blender_asset_tracer/trace/file_sequence.py

@@ -56,7 +56,10 @@ def expand_sequence(path: pathlib.Path) -> typing.Iterator[pathlib.Path]:
         raise DoesNotExist(path)
 
     if path.is_dir():
-        yield path
+        # Explode directory paths into separate files.
+        for subpath in path.rglob("*"):
+            if subpath.is_file():
+                yield subpath
         return
 
     log.debug("expanding file sequence %s", path)