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

xmodule/js/spec/annotatable/display_spec.js

@@ -0,0 +1,10 @@
+describe('Annotatable', function() {
+    beforeEach(() => loadFixtures('annotatable.html'));
+    describe('constructor', function() {
+        const el = $('.xblock-student_view.xmodule_AnnotatableModule');
+        beforeEach(function() {
+            this.annotatable = new Annotatable(el);
+        });
+        it('works', () => expect(1).toBe(1));
+    });
+});