text: Title: Rename DAG (from models/dag) to SchedulerDAG -- or remove it!
  Body: ### 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.