Improve Maven/CodeGenerator plugin interaction

Current yang-maven.plugin.spi does not isolate plugins from interactions with files in the project, which means we cannot guarantee interactions with external build lifecycles which support incremental builds.

Unfortunately there is no unified incremental build API which works for both Eclipse and command-line out of the box, so the simple idea of propagating that to codegen plugins will not work.

We therefore have to define an independent isolated API, by which the generators express their intent to generate files and have yang-maven-plugin core integrate with lifecycle as appropriate.

Define sucn an API and make sure yang-maven-plugin takes advantage of its facilities.