(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'm surprised that this works; this is using rspec mocks. In Puppet we use mocha mocks, so this should be `resource.provider.expects(:update)`