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).
Perhaps this test should be stronger and assert that the text found is preceded/followed by either a newline or the beginning/end of the string. This would allow you to detect e.g. improper merging of dependencies.