Support multiple file dependencies with --setup_file option

[cisaacstern]

Users may want to put some code in modules outside their recipe module. To access such code, a setup file is needed, per beam docs:

https://beam.apache.org/documentation/sdks/python-pipeline-dependencies/#multiple-file-dependencies

@jbusecke recently ran into this issue here.

Haven't quite wrapped my head around how much this path will diverge from what we're currently doing (maybe not too much, maybe a bit).

[yuvipanda]

@cisaacstern I actually got most of this done many months ago. Needs rebasing and completion though. https://github.com/pangeo-forge/pangeo-forge-runner/compare/setup_py?expand=1