(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.
I suppose I don't know what you're referring to.  If you mean that we have self here, I thought I needed to write a class method to call rather than an instance method, since it didn't appear that at the time instances was called there was not an actual instance.