Right now, when a service is not found, we throw a ServiceNotFoundException with some suggestions.
But when the lookup is for a private service that ended up being removed or inlined, that message is not accurate: we should instead hint about the service being private + suggest to either turn it public or not use the container at all (similar to the deprecation in Container::get()).

On 4.0, where services will be private by default, this looks like a DX requirement to me, so we should do it during the feat. freeze.