Currently pipeline-model-definition has a dependency on docker-workflow to supply agent docker and agent dockerfile definitions. These should rather be defined in the docker-workflow plugin, which would then get a dependency on pipeline-model-extensions (unclear if that should be optional).
This would match, for example, how agent kubernetes is defined in the kubernetes plugin, and how I propose (related to JENKINS-48050) something like agent dockerNode should be defined in the docker plugin, keeping the sensitive aspects of the behavior close to the plugin which manages that environment.
It would also allow docker-workflow to be omitted from distributions which do not need it. In particular, Kubernetes-oriented distributions are unlikely to have any use for agent docker. (You might want to install docker-workflow so as to be able to use withDockerContainer with DinD, but in that case you would still be using agent kubernetes, and anyway the most common use case—building Docker images—is preferably handled via Kaniko.)