This re-applies commit 36cc415 with handling an invalid context_key in the
`PartitionService`. It can happen when rendering a `LibraryContentBlock` in
Studio because this service is initialized by the modulestore when validating
an XBlock to gather its error messages in the `studio_xblock_wrapper`.
- 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
