(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.
This requires the current working directory to be in the root of the directory, which is brittle. We have a set of methods for including using fixtures, the openbsd provider (https://github.com/puppetlabs/puppet/blob/master/spec/unit/provider/package/openbsd_spec.rb#L69) shows how they can be used.