[desktop_drop] Exporting 'events.dart' request

[13thdeus]

I was working on drop zone highlighting. I used a raw drop event listener to change the widget state when the user starts dragging a file over the application. I had to import a file from the src directory to complete my task.

//Linter message: Import of a library in the 'lib/src' directory of another package. (Documentation)
import 'package:desktop_drop/src/events.dart';
...
  @override
  void initState() {
    DesktopDrop.instance.addRawDropEventListener(_onDropEvent);
    ...
    super.initState();
  }

  @override
  void dispose() {
    DesktopDrop.instance.removeRawDropEventListener(_onDropEvent);
    ...
    super.dispose();
  }

  void _onDropEvent(DropEvent event) {
    if (_dropSubject.value && (event is DropDoneEvent || event is DropExitEvent)) {
      _dropSubject.add(false);
    } else if (!_dropSubject.value && event is DropEnterEvent) {
      _dropSubject.add(true);
    }
  }