public static VirtualSourceResolver of() {
        return new VirtualSourceResolver(Map.of());
    }
Multiple sources like contentcluster.schema1, contencluster.schema2 needs to respond
when source is the prefix contentcluster. This is done by generating map from virtual source
to the fully qualified ones, and resolving from there.

@author baldersheim
container-search/src/main/java/com/yahoo/search/federation/sourceref/VirtualSourceResolver.java