New analyzer + code fix: Make DI implementations internal

[ronnygunawan]

public interface IService { }

public class Service : IService { }

public class AnotherService { }

services.AddTransient<IService, Service>(); // warning: 'Service' is already registered to service collection
                                            // through its interface 'IService' and should not be exposed
                                            // code fix: Make 'Service' internal

services.AddTransient<AnotherService>(); // no warning