text: Title: WiringConfiguration(packages=[top_level_package]) affect subpackages only if they have __init__.py
  Body: Hello folks! I'm just started to learn dependency-injector.
  I was following this public template [https://github.com/teamhide/fastapi-boilerplate](https://github.com/teamhide/fastapi-boilerplate) and found that WiringConfiguration can be affect subpackages of designated top level package only if all of paths have __init__.py.
  
  I understand that from Python 3.6 we don't necessarily need to use __init__.py not if in some of situation but i believe this might not be one of the those cases.
  
  **app.container.py**
  ```python
  from dependency_injector.containers import DeclarativeContainer, WiringConfiguration
  from dependency_injector.providers import Factory, Singleton
  
  from app.auth.application.service.jwt import JwtService
  from app.user.adapter.output.persistence.repository_adapter import UserRepositoryAdapter
  from app.user.adapter.output.persistence.sqlalchemy.user import UserSQLAlchemyRepo
  from app.user.application.service.user import UserService
  
  
  class Container(DeclarativeContainer):
      wiring_config = WiringConfiguration(packages=["app"])
  
      user_repo = Singleton(UserSQLAlchemyRepo)
      user_repo_adapter = Factory(UserRepositoryAdapter, user_repo=user_repo)
      user_service = Factory(UserService, repository=user_repo_adapter)
  
      jwt_service = Factory(JwtService)
  ```