Allow to extend a service which is not defined yet.

I'm confronted with a case where I must extend a service which is not definied yet (when all services are set directly in an array in the constructor for exemple). In one hand, I think it's a good thing to forbid to extend a service which doesn't exist, but in the other hand, according to the Pimple documentation, "the order of the definitions does not matter". So I propose this PR that move the check of the identifier (exist and is callable) from the extend method to the extended closure.