private static Set<String> set(String successor) {
            return successor == null ? null : new LinkedHashSet<>(List.of(successor));
        }
Creates the search chains vespaPhases, vespa and native.

@author Tony Vaagenes
config-model/src/main/java/com/yahoo/vespa/model/container/search/searchchain/defaultsearchchains/VespaSearchChainsCreator.java