DI 2.0.0

Seems like depth is internal only (to track circular deps).

What about naming it `_depth` and a a comment.

It might be even better not to expose this to the public API.

It should be possible with:

```dart
class ModuleInjector {
int depth;

dynamic getByKey(Key key) {
depth = 0;
_getByKey(Key key)
}

dynamic _getByKey(Key key) {
if (depth++ > 42) throw;
//...
}
```