A number of old utility functions and classes have been removed:

``MapReduceMixin``: To make a checker reduce map data simply implement
``get_map_data`` and ``reduce_map_data``.

``is_inside_lambda``: Use ``utils.get_node_first_ancestor_of_type(x, nodes.Lambda)``

``check_messages``: Use ``utils.only_required_for_messages``

``is_class_subscriptable_pep585_with_postponed_evaluation_enabled``: Use
``is_postponed_evaluation_enabled(node)`` and ``is_node_in_type_annotation_context(node)``

``get_python_path``: assumption that there's always an __init__.py is not true since
python 3.3 and is causing problems, particularly with PEP 420. Use ``discover_package_path``
and pass source root(s).

``fix_import_path``: Use ``augmented_sys_path`` and pass additional ``sys.path``
entries as an argument obtained from ``discover_package_path``.

``get_global_option``: Use ``checker.linter.config`` to get all global options.

Related private objects have been removed as well.

Refs #8409