Helper generator that traverses modules in returns a flattened
    iterator.

def _walk_modules(modules, class_name, path, ignored_formats, args):
    """
    Helper generator that traverses modules in returns a flattened
    iterator.
    """
    for module in _iter_modules(modules=modules,
                                class_name=class_name,
                                path=path,
                                ignored_formats=ignored_formats,
                                args=args):
        for section in module.sections:
            for lecture in section.lectures:
                for resource in lecture.resources:
                    yield module, section, lecture, resource