HV-828 Introducing ConstraintValidatorLocator to allow for a pluggable d...

...iscovery of additional constraint validator implementations.
> Btw. how do this new feature and XML-configured constraints play together, in particular with regards to contradictory "include existing validators" configurations? Is it determined which one "wins" in this case?

If either XML or a locator removes the default constraint validators, they are getting removed. No matter whether another locator or the XML config says, they should stay. I am not sure it is worth the effort to try to be smarter for now. It would also require some sort of priority. WDYT?