Move this into its own module to  keep from having to import from conda.cli in conda-env
Refactor to provide a better API into this functionality

This provides a dict-like object containing information about a
pip-installed package that can be cast to a string to show its
representation.  This should eventually be fleshed out a bit more.
Add test around returning from generator

I wasn't sure on the behavior of return inside a generator that yields
so I wanted to test to verify that an early return was valid.
Fix logic

This skips any packages that were installed by conda *and* has a pip_pkg
without a path.  The path indicates a locally installed package via
`develop` that should always be included.
Merge pull request #1015 from tswicegood/refactor/pip-list

Refactor the pip code around conda list