(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 don't actually think this code is used due to missing the ability to accept an argument, though this reminds me that its not clear what the bare minimum to implement `:versionabe` is.  I'll try to make sure the tests cover this case, and test if I can remove it.