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/s3.py

@@ -123,7 +123,7 @@ class S3Transferrer(transfer.FileTransferer):
         return True
 
     def _upload_callback(self, bytes_uploaded: int):
-        if self.abort.is_set():
+        if self._abort.is_set():
             log.warning('Interrupting ongoing upload')
             raise self.AbortUpload('interrupting ongoing upload')