Remove legacy repo format from loading path
Summary: This is the breaking change part of https://dagster.phacility.com/D4742 that actually removes the legacy format from the loading path and starts erroring instead of warning. We can sit on this until just before the 0.10.0 release.
Test Plan: BK+Azure
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D4824