vlc_modules: add an API to get the shortcut to use with a module

A plugin may contain multiple modules. They all have the same first shortcut which is the VLC_MODULE_NAME [1]. Then if there's a shortcut (or more) it's further in the list.

module_get_object() cannot differentiate between the modules inside a plugin. It should keep returning the name of the VLC_MODULE_NAME. But we need another API to get the proper shortcut to use.

This is especially necessary in 3.0 to solve https://forum.videolan.org/viewtopic.php?f=14&t=164879. Except we probably can't add a libvlccore API there.

Ref. #28691