Use PKG-INFO and METADATA to infer package names in pip analysis

[nkubala]

Fixes #287

This code adds improvements to the pip analyzer to produce more accurate results for installed Python packages, by using required (read: encouraged) metadata files in egg and wheel packages.

Some packages do not always adhere to these conventions, so in the event we can't use the expected metadata files, we fall back on the original heuristic of checking directories with similar names and assuming those are the corresponding directories. This code slightly modified this heuristic from checking the "previous alphabetical entry" to checking the previous and next entries, and doing a string comparison to match the package name. This gives more accurate results, for example with egg bzr-2.8.0dev1.egg-info and matching code directory bzrlib.

Note: this code modifies results from container-diff to include all installed python packages, not just those explicitly installed through pip. This means the results will be different than what is retrieved by running pip list, but gives more insight into what dependencies have been installed.