(PUP-2452) Reinstall support for Package type

This is implemented behind a feature that each provider must implement. It is intended to allow source distributions to recompile packages on a notify or subscribe event.

It allows patterns like:

    package { "www-servers/nginx":
       ensure    => latest,
       subscribe => File['/path/to/nginx/compile/options'];
    }
Okay, I think I'm done re-editing that now. :)