Add conda support

[carmilso]

This PR allows to find pip differences when there is a Conda installation. As a data scientist I want to find pip differences when I use an image that comes with Conda (i.e.: all those related to jupyter notebook). For doing this now we look not only for variable PYTHONPATH but also for variable CONDA_DIR.

Example of image that uses CONDA_DIR: https://hub.docker.com/r/jupyter/minimal-notebook

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[carmilso]

@googlebot I signed it!