(PUP-2452) Refresh 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 { "app-admin/nginx":
       ensure       => latest,
       hasreinstall => true,
       subscribe    => File['/path/to/nginx/compile/options'];
    }