private void addDependency(ModelNode node) {
        dependencies.put(node.id, node);
    }
This adds dependencies base on constructor arguments in the model classes themselves.
These then have to be created by this mini-di framework and then handed to the builders
that will fill them.

TODO: This should be changed to model dependencies between model builders instead, such
that they can create their model objects, and eventually make them immutable.
config-model/src/main/java/com/yahoo/config/model/graph/ModelNode.java