Double-exported tasks aren't visible to grinder

[nex3]

If grind.dart exports a file that defines a task, grinder will see that task. However, if that file in turn exports another file that defines a task, grinder won't see it. This makes it difficult to write libraries that provide tasks that users can include in their grind.dart via export (which is the only means available to do so as far as I know).

For example:

// grind.dart
import 'package:grinder/grinder.dart';

export 'midstream.dart';

main(List<String> args) => grind(args);

// midstream.dart
export 'upstream.dart';

// upstream.dart
import 'package:grinder/grinder.dart';

@Task('Print hello.')
hello() {
  print("hello");
}