refactor executors into standalone package

[rabernat]

Currently our executors are just imported from rechunker:

https://github.com/pangeo-forge/pangeo-forge/blob/5e2362acdd77a5c4441e8ab57deca6577bbd4dff/pangeo_forge/executors.py#L1-L3

We probably want to move the executors into a standalone package.

This means moving these three executor classes:

• https://github.com/pangeo-data/rechunker/blob/master/rechunker/executors/python.py
• https://github.com/pangeo-data/rechunker/blob/master/rechunker/executors/dask.py
• https://github.com/pangeo-data/rechunker/blob/master/rechunker/executors/prefect.py

out of rechunker, along with their tests:

• https://github.com/pangeo-data/rechunker/blob/master/tests/test_pipelines.py

This is a bit of a chore (setting up new repo, CI, docs, packaging, etc.) But in the long term I think it is really the right way to go. This way they can be used by both packages without messy interdependencies.

[rabernat]

We are not going to do this anymore. This is superceded by #160.