Support nested `$overrides`

[nfroidure]

Currently is is impossible to override a service and its dependencies at the same time :

export default {
  sendApplicationMessage: 'sendSlackMessage',
  sendApplicationMessage: {
    SLACK_CONFIG: 'SLACK_APPLICATION_CONFIG'
  }
}

Won't work since the keys are duplicated. We should support the following syntax:

export default {
  sendApplicationMessage: {
    __self: 'sendSlackMessage',
    SLACK_CONFIG: 'SLACK_APPLICATION_CONFIG'
  }
}