Allow aborting a pack operation.

For this to work well I also had to remove the sorting of blocks in
trace.deps(). The sorting caused the first `yield` to be executed only
after each blend file was opened, which means that the consuming for-loop
takes a long time to hit its first iteration. As a result, it would respond
slowly to abort requests. By not sorting the first `yield` is much sooner,
resolving this issue.

blender_asset_tracer/trace/__init__.py

@@ -32,10 +32,7 @@ def deps(bfilepath: pathlib.Path, progress_cb: typing.Optional[progress.Callback
     if progress_cb:
         bi.progress_cb = progress_cb
 
-    ahb = asset_holding_blocks(bi.iter_blocks(bfile))
-    # Sort the asset-holding blocks so that we can iterate over them
-    # in disk order, which is slightly faster than random order.
-    for block in sorted(ahb):
+    for block in asset_holding_blocks(bi.iter_blocks(bfile)):
         yield from blocks2assets.iter_assets(block)