It doesn't make sense to have every stage that processes some aspect of SAML metadata (including the many extension specifications) all thrown in to one package. When I introduced the RegistrationAuthorityPopulationStage I therefore created a new mdrpi package for that particular extension.

It would make sense to move the existing EntityRegistrationAuthorityFilterStage down into that package, as well as the namespace constant it uses which is currently in SAMLSupport; this information is also available in mdrpi.MDRPISupport.