Restructure: Rename buildpack to cpython and modify the API

[Arjun Sreedharan]

Background

See RFC 0001 that outlines the proposed restructure of python family of buildpacks.

Proposal

• This repository should be renamed to cpython.
• Buildpack API should be as follows:
  • provides: cpython OR python-runtime python
  • requires: none

Note: The python-runtime python provision is temporary and is intended to facilitate a smooth transition to the new structure/api.
See an example of OR-ed bulidplan here.

EDIT: Fix s/python-runtime/python for the API provision.