Integrate YANG parser with XPath parser

We now have a custom XPath parser which allows extension and introspection. This allows us to update RevisionAwareXpath interface to expose a properly-parsed XPath, without the need for users to muck with strings and modules.

Integrate yang-xpath-api into yang-model-api and extend YANG parser to properly parse XPath strings. Also expose an option for performing lenient parsing, so that users can fall back to old behavior should there be any bugs.