We should get rid of GetConstraintValidatorsFromServiceLoader and use GetInstancesFromServiceLoader instead.

Moreover, I think we can safely remove serviceLoaderBasedConstraintMappingContributor from ConfigurationImpl.

We can instantiate it in ValidatorFactoryImpl: it will be safer (we will need to pass the externalClassLoader to it and thus it will be possible to call getServiceLoaderBasedConstraintMappingContributor() before the call to externalClassLoader(...) which would be wrong) and we don't need to keep it around, once the initialization is done.