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

.github/actions/verify-tests-count/action.yml

@@ -7,7 +7,7 @@ runs:
       shell: bash
       run: |
         echo "root_cms_unit_tests_count=$(pytest --collect-only --ds=cms.envs.test cms/ -q | head -n -2 | wc -l)" >> $GITHUB_ENV
-        echo "root_lms_unit_tests_count=$(pytest --collect-only --ds=lms.envs.test lms/ openedx/ common/djangoapps/ common/lib/ -q | head -n -2 | wc -l)" >> $GITHUB_ENV
+        echo "root_lms_unit_tests_count=$(pytest --collect-only --ds=lms.envs.test lms/ openedx/ common/djangoapps/ common/lib/ xmodule/ -q | head -n -2 | wc -l)" >> $GITHUB_ENV
 
     - name: get GHA unit test paths
       shell: bash