PTFM-12951 Move EE dependency installation code into dxpy

@psung, please review.

This is the SDK portion of the move. The EM/EE portion will involve deleting the EM code and changing dx-prepare-exec-env to use this code.

Also some minor formatting changes, and a change to dx-log-stream to remove code that is unneeded after dx-exec-helper changes (which will land together with the other EM/EE changes above).
We should consider not grouping pip dependencies, since such grouping can cause the packages to not be installed correctly-- https://github.com/pypa/pip/issues/1386