PIM-3544 : first commit with copy / paste denormalizers
PIM-3544 : change namespaces, declare and register new services
Merge pull request #1898 from akeneo/PIM-3544

PIM-3544 : variant groups - flat denormalizers
PIM-3541 : introduce an interface for Product Group entity
PIM-3541 : use the new GroupInterface
PIM-3541 : fix CS