Tasks can't depend on re-assigned task names

The following fails with ```Task `after` references invalid task method `before` as a dependency```:

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

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

void before() => print("before");

@Task('')
const beforeTask = before;

@Depends(beforeTask)
@Task('')
void after() {}
```

Assigning one task to another would otherwise be one way to work around #337.