ModelResolver modelResolver = new ExtensibleModelResolver(contribution, modelResolvers, modelFactories);
contribution.setModelResolver(modelResolver);
contribution.setUnresolved(true);
DefinitionsFactory definitionsFactory = modelFactories.getFactory(DefinitionsFactory.class);
Definitions definitions = definitionsFactory.createDefinitions();
DefinitionsUtil.aggregate(systemDefinitions, definitions, monitor);
// create an artifact to represent the system defintions and
// add it to the contribution
List<Artifact> artifacts = contribution.getArtifacts();