Refactor xml_importer.py for easier reading.

cms/djangoapps/contentstore/management/commands/import.py

cms/djangoapps/contentstore/tests/test_contentstore.py

cms/djangoapps/contentstore/tests/test_import.py

cms/djangoapps/contentstore/tests/test_import_draft_order.py

cms/djangoapps/contentstore/tests/test_import_pure_xblock.py

cms/djangoapps/contentstore/views/import_export.py

cms/djangoapps/contentstore/views/tests/test_assets.py

common/lib/xmodule/xmodule/modulestore/xml_importer.py