Most of the methods in `XmlMixin` act as wrappers for the official API for
serialization and deserialization (parse_xml() and add_xml_to_node()).
`XmlParserMixin` contains the code which does the actual serialization and
deserialization.
- 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
