We've been gradually bloating with more and more extension points, largely in pipeline-model-definition. Meanwhile, we've got pipeline-model-declarative-agent which contains the Declarative agent extension point and related stuff. So let's collect all the extension points into one plugin and have pipeline-model-definition depend on that plugin (which we're naming pipeline-model-extensions for now, at least) instead of pipeline-model-declarative-agent. Finally, let's deprecate/tombstone pipeline-model-declarative-agent before anything else out there depends on it. =)

This is aimed at 1.1.