* fixed problem with creating some CourseItem with some parent:
we must get the updated parent item before using that parent item
* replaced parent_location with parent argument in ItemFactory due to
error children/parent relation for split modulestore. In all tests with
split modulestore parent argument used
- 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
