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/fixtures/html-edit-visual.html

@@ -0,0 +1,10 @@
+<div class="test-component">
+    <div class="wrapper-comp-editor" id="editor-tab" data-base-asset-url="/c4x/foo/bar/asset/" data-editor="visual">
+        <section class="html-editor editor">
+            <div class="row">
+                <textarea class="tiny-mce"><p>original visual text</p></textarea>
+                <textarea name="" class="edit-box">raw text</textarea>
+            </div>
+        </section>
+    </div>
+</div>