Pub/Publish: Warn when you are using an import transitively

[matanlurey]

I'd be OK with this an optional flag, but I really think it should be the default.

i.e., if I have in my pubspec:

dependencies:
    angular2:

I will get a .packages resolution that includes many things, like source_span.

If I try to use source_span without relying on it, I'd like to see this become a warning/error:

// Hehe! I can use this because Angular does!
import 'package:source_span/source_span.dart';

[nex3]

Duplicate of #343.