don't create an extra assignment on import in draft state

test plan:
* import the package referenced in the ticket into a
 draft-state enabled course
* should not have extra assignments created in the
 default "Assignments" group

closes #CNVS-13345

Change-Id: I7debc455014b5a44fe2026d85de8b58ad4ae89fb
Reviewed-on: https://gerrit.instructure.com/38673
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>