project_verify_package should optionally validate drupal-org-core.make files

Given that these are now live:
#1455614: Packaging script doesn't allow distributions to patch core or specify a git revision
#1469714: Fix validation to support drupal-org-core.make files and prevent defining the drupal project in drupal-org.make itself
we should probably teach project_verify_package about them so that if a drupal-org-core.make file exists for the release people are trying to create, we also verify that.