Rename DAG (from models/dag) to SchedulerDAG -- or remove it!

### Body It is easily confused with the DAG in task sdk. Ideally we can remove. At least rename it. comment from ash > That should be renamed as SchedulerDAG really -- (or possibly it can be removed now) -- it's a) a compat import while that part isn't complete, and b) where scheduler specific DAG functions go ### Committer - [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.