private static Cluster clusterOfSource(String source, Map<String, Cluster> clusters) {
            var cluster = clusters.get(source);
            if (cluster != null) return cluster;
            for (var c : clusters.values()) {
                if (c.schemas().contains(source))
                    return c;
            }
            return null;
        }
A source name is either a cluster or a schema.
Returns the cluster which either is or contains this name, if any.
container-search/src/main/java/com/yahoo/search/schema/SchemaInfo.java