AppContextProviderLookup

AppContextProviderLookup is a helper trait used by AppContext[T] to search for providers with correct priority: AppContextProviders in enclosing scope takes precedence over AppContextProvider[T] defined in T's companion object.

This works because implicit search looks in the companion of the result type (T), but AppContextProviderLookup[T]'s companion is AppContextProviderLookup, not T. So givens defined here (fromProviders) have priority over T's companion.