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

scripts/unit-tests.sh

@@ -116,7 +116,7 @@ case "${TEST_SUITE}" in
                 mv reports/${TEST_SUITE}.coverage reports/.coverage.commonlib
                 ;;
             [1-2])
-                paver test_lib -l common/lib/xmodule --disable_capture --eval-attr="shard==$SHARD" ${PAVER_ARGS} 2> common-tests.${SHARD}.log
+                paver test_lib -l ./xmodule --disable_capture --eval-attr="shard==$SHARD" ${PAVER_ARGS} 2> common-tests.${SHARD}.log
                 mv reports/${TEST_SUITE}.coverage reports/.coverage.commonlib.${SHARD}
                 ;;
             3|"noshard")