importer refactor: yield missing links instead of going through opts

test plan:
* nothing should have changed

Change-Id: I6e7e26dfff5c3239771f4321de91b7d0f28c9845
Reviewed-on: https://gerrit.instructure.com/36487
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>