Plugin class_name type hints and singleton dependency loop

[arccoza]

Describe the bug When you have a class in a plugin that depends on a Singleton loaded by that plugin (with add_autoload_singleton) you can end up in a dependency loop very easily. See: https://github.com/godotengine/godot/issues/42651

To Reproduce Steps to reproduce the behavior:

1. Create a plugin.
2. Use add_autoload_singleton in the class to load a singleton.
3. Create a class that depends on the singleton.
4. Use that class as a type hint in the plugin script.

Minimal reproduction project: plugin_class_singleton_dep_loop.zip

[arccoza]

Related: #2 #5 #7