Use `require_relative` for all components under Puppetx. This makes it possible
to load `pupetx/yardoc/yard/plugin` outside of Puppet as a YARD plugin.
Also fix the `yardoc` face so that Puppetx bits are loaded _after_ feature
checks.
The `YARDStatement` class is an adaptor that composes a `Pops::Model` instance
along with applicable `Pops::Adapter` and presents a familiar interface for
`YARD::Handler` objects to work with.
Start building a YARD parser. Parsing and comment extraction has been moved out
of the yardoc face and into the new `Puppetx::Yardoc::YARD::PuppetParser`
class. The old `Commentor` class from the Util module has been renamed to
`Puppetx::Yardoc::Pops::YARDTransformer`.
`puppet yardoc` is still capable of extracting docstrings for node, class and
type definitions from a single manifest file.
Charlie Sharpsteen