Various tweaks to pip downloading

- Move the pip dependency downloading function + logic
  to the pip_helper module instead of being in the base.py
  module (since its a pip utility function)
- Add a retry function to utils and use it instead of the
  custom retry module used for downloading pip dependencies.
- Cleanup other various code in pip_helper.py

Change-Id: I038e4d73c814ea06a18eae61e41c5b7bef779b9d
Merge "Various tweaks to pip downloading"