Features/memaxes

- packages can now extend only one other package.
- do_activate() and do_deactivate() are now called on the extension,
  and they automatically find the extendee
- activate() and deactivate() are still called on the extendee and are
  passed the extension.

- lets packages do some setup before their extensions run install()

- packages can be "extended" by others
- allows extension to be symlinked into extendee's prefix.
- used for python modules.
  - first module: py-setuptools

Mitos package

- not used until optional/conditional deps are implemented.