Attaching a work in progress file which intend to replace the current install.py file. The implementation isn't finished yet but the overall design is here.

It comes with four classes:

- Installer which manages the overall installation procedure. It shares a distribution cache with the dependency handler so distributions are not downloaded / extracted multiple times. The installer is able to install / remove single distributions OR distributions with their dependency tree.
- DependencyHandler which takes care about getting the dependencies from either the metadata stored in the indexes or inside the distirbutions.
- The DistributionCache class is just a store containing the location of the distributions that had been extracted/downloaded. It provides methods to ease the work with them (so any external user don't need to care about the state of the distribution when requesting it).
- DirectoryMover provides a way to deal with moving/deleting operations with a commit/rollback API. The goal is to simplify a lot the current API and to provide a reusable tool for other installers. why not putting this in shutil at some point.

This file is provided here mostly for feedback, but the work is not finished yet.