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/pack/progress.py

@@ -23,6 +23,9 @@ class Callback(blender_asset_tracer.trace.progress.Callback):
                   missing_files: typing.Set[pathlib.Path]) -> None:
         """Called when packing is done."""
 
+    def pack_aborted(self):
+        """Called when packing was aborted."""
+
     def trace_blendfile(self, filename: pathlib.Path) -> None:
         """Called for every blendfile opened when tracing dependencies."""