public SourceGroup getComponent(ComponentSpecification spec) {
        ComponentAdaptor<SourceGroup> result = sourceGroups.getComponent(spec);
        return (result != null)?
                result.model :
                null;
    }
Owns all the source groups in the search chains model.
@author Tony Vaagenes
config-model/src/main/java/com/yahoo/vespa/model/container/search/searchchain/SourceGroupRegistry.java