'lookup_prefix' on SimpleRouter.get_lookup_regex, easing code de-duplication

This allows @alanjds/drf-nested-routers to not duplicate/monkeypatch work made here. Without retro-compatible patch, updates on SimpleRouter.get_lookup_regex will need to be copied-over to drf-nested-routers, violating DRY. Also, all router needed a "nested" version will probable need to be copied-over too.

Related to #1048