translate sis source_course_ids in migration api

test plan:
 - use the API to copy a course referred by sis_source_id

fixes CNVS-10959

Change-Id: I2297e86969dbc630f75a6ce856a3603b919fd62b
Reviewed-on: https://gerrit.instructure.com/29795
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>