(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,
       subscribe => File['/path/to/nginx/compile/options'];
    }
Minor style nitpick: if you put the && at the end of the preceding line, the line continuation character won't be needed.