private List<FederationSearcherModel.TargetSpec> readSources(Element searcherSpec) {
            List<FederationSearcherModel.TargetSpec> sources = new ArrayList<>();
            for (Element source : XML.getChildren(searcherSpec, "source")) {
                sources.add(readSource(source));
            }
            return sources;
        }
Builds a federation searcher config producer from an element.

@author Tony Vaagenes
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/chains/search/DomFederationSearcherBuilder.java