(PUP-3618) Include pkgng provider for FreeBSD

To improve the default state of Puppet on FreeBSD systems, !his work
moves the pkgng provider into core.  Previously, users of the provider
would need to install from the forge module zleslie/pkgng.
The `@property_hash` should have everything we care about at this point, so I'm not sure why I'd need to inspect the system again here unless I misunderstand (likely) the order of operations here.