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

lms/djangoapps/grades/tests/integration/test_problems.py

@@ -5,6 +5,8 @@ import itertools
 import ddt
 import pytz
 from crum import set_current_request
+from capa.tests.response_xml_factory import MultipleChoiceResponseXMLFactory
+
 from xmodule.graders import ProblemScore
 from xmodule.modulestore import ModuleStoreEnum
 from xmodule.modulestore.tests.django_utils import (
@@ -13,8 +15,6 @@ from xmodule.modulestore.tests.django_utils import (
 from xmodule.modulestore.tests.factories import CourseFactory, ItemFactory
 from xmodule.modulestore.tests.utils import TEST_DATA_DIR
 from xmodule.modulestore.xml_importer import import_course_from_xml
-
-from capa.tests.response_xml_factory import MultipleChoiceResponseXMLFactory
 from common.djangoapps.student.models import CourseEnrollment
 from common.djangoapps.student.tests.factories import UserFactory
 from lms.djangoapps.course_blocks.api import get_course_blocks