fix autoloading of the Importers module

It wasn't following the rails file naming convention

fixes CNVS-14360

Test plan: imports, such as state standards imports, should work even
without class caching

Change-Id: I99ac8448844b7ac852203a5b8127d6fa35ea5f12
Reviewed-on: https://gerrit.instructure.com/38090
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>