improve cc 1.3 support

test plan:
* import the test assignment fixtures as a common cartridge file
* there should be an assignment in the course

* also test import of flat manifest xml files

closes #CNVS-5906 #CNVS-13219

Change-Id: I0b765ed0d140516f78a5f4fb5dbe9b81ac77ecd4
Reviewed-on: https://gerrit.instructure.com/35394
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>