Add support for the serviceName (keyed services) in the MEF and Autofac adapters.

[ioancrisan]

It would be great to just import as service with a provided name, like JSON serializer.
Prerequisites are here: ServiceNameAttribute and serviceName parameter in ICompositionContext.
For MEF/Autofac it is missing the named registration, for Autofac the retrieval over serviceName.
Open issue: how to neutralize the flavor when importing over parameters?