Allow access to the Cluster's subscribe(), unsubscribe(), publish(),
publish_hrw() and publish_rr() methods by loading only the
base/frameworks/cluster/pubsub, rather than everything that
__load__.zeek or also main.zeek pulls in.
This can be used by other scripts to use these functions without
relying or expecting the rest of the cluster framework to be loaded
already. Concretely, this is needed to move the Supervisor framework
from Broker to Cluster.
