bloc_library example: dependency between BLoC classes

[`bloc_library`](https://github.com/brianegan/flutter_architecture_samples/tree/main/bloc_library) example contains direct dependency between `FilteredTodosBloc` and `TodosBloc`. It is directly not recommended in BLoC library [documentation](https://bloclibrary.dev/architecture/#bloc-to-bloc-communication). Please fix it.

https://github.com/brianegan/flutter_architecture_samples/blob/d898d1329e04e5b5fbdef1285b39ef975a6b8efa/bloc_library/lib/blocs/filtered_todos/filtered_todos_bloc.dart#L12-L25

Start with bloc_library/lib/blocs/filtered_todos/filtered_todos_bloc.dart at the linked dependency and read the BLoC-to-BLoC communication guidance in the linked documentation. Trace how FilteredTodosBloc currently obtains data from TodosBloc, then refactor the example to remove that direct dependency while preserving filtered-todo behavior.

Written by the indexing model from the issue text.