Hide use of Handle by edm::RefProd from Reflex

When building the recent change to edm::Handle, one package failed because Reflex decided to instantiate all constructors for the RefProd during parsing. Given edm::Handle is only forward declared for Reflex this failed. Strange thing is many other packages instantiated dictionaries for edm::RefProd without a problem. This change hides the implementation of the problematic constructor from RefProd.