[DF] Rename dask_planner.rust Python module to dask_sql.planner

[jdye64]

Is your feature request related to a problem? Please describe. Rename dask_planner.rust Python module to dask_sql.planner

Describe the solution you'd like Be able to import the Rust module as dask_sql.planner to make project structure more clear

Describe alternatives you've considered None

Additional context None