Add service containers to Betty in order to provide a single solution to managing service dependencies, rebuilds, and service visibility/scope/context, e.g. a service can be available for Projects but not for Apps.

Goals

• Provide a single API for declaring and getting services
• Gather dependencies between services
• Guard for cyclic dependencies
• Allow services to be rebuilt dynamically if their dependencies are rebuilt
• Each dependency factory should receive a service container builder that lets them fetch other services (dependencies) and register additional rebuild callbacks
• A service container can wrap another, allowing additional services to be declared, or existing ones to be overridden without impacting the wrapped container. This lets us have base app services, and allows projects to make additional ones available through extensions.
• Services must be lazily instantiated when they are first requested
• Provide thread-safety through locks where needed