refactor: move xmodule folder to root

- Moving xmodule folder to root as we're dissolving sub-projects of common folder in edx-platform
    - More info: https://openedx.atlassian.net/browse/BOM-2579
- -e common/lib/xmodule has been removed from the requirements as xmodule has itself become the part of edx-platform and not being installed through requirements
- The test files common/lib/xmodule/test_files/ have been removed as they are not being used anymore

openedx/core/djangoapps/content/learning_sequences/api/processors/enrollment_track_partition_groups.py

@@ -44,7 +44,8 @@ class EnrollmentTrackPartitionGroupsOutlineProcessor(OutlineProcessor):
             self.user,
             partition_dict_key='id'
         )
-        self.user_group = self.enrollment_track_groups.get(ENROLLMENT_TRACK_PARTITION_ID)
+        # TODO: fix type annotation: https://github.com/openedx/tcril-engineering/issues/313
+        self.user_group = self.enrollment_track_groups.get(ENROLLMENT_TRACK_PARTITION_ID)  # type: ignore
 
     def _is_user_excluded_by_partition_group(self, user_partition_groups):
         """