Protect some services from being referenced from outside container

Problem/Motivation

Services can be declared as non public in their definition. A service marked like this cannot be accessed with $container->get('service-id');, rather it can only be referenced from other services. Also note that...

If you use a private service as an argument to only one other service, this will result in an inlined instantiation (e.g. new PrivateFooBar()) inside this other service, making it publicly unavailable at runtime.

Proposed resolution

1. Determine whether inlining of non-public services results in substantial performance gains.
2. Agree on a policy defining which service classes should be marked non-public. Document that policy.
3. Go through all service definitions and apply the policy.

Remaining tasks

User interface changes

API changes