Refactor the pip code around conda list 

Was reading through the tonight and saw some code that could use cleaning up, so I cleaned it up.  Functionally this is the same, though all of the unit tests are around logic that was extracted and nothing around `conda.cli.main_list`.  There were no tests specifically for that, and still aren't.  What I did to test was install from master and do the following:

```bash
$ conda list conda | wc -l
11
$ git checkout refactor/pip-list
... git checkout output
$ conda list conda | wc -l
11
```

I've also done a comparison diff between the full output from master and this branch and there are no differences.