plugins – Loading class files via autoload method

We have core classes created for theme and we are moving those classes to client-mu-plugins. Is there any way we can load all of the classes without adding require_once function for every single files? I tried spl_autoload_register() function in every single ways but It’s not loading.

File structure in plugin now

plugin_folder
  -inc
      - frontend
         -class-post.php
         -class-design.php
         -class-video.php
      - modules
         -class-query.php
         -class-config.php
      - queries
  - plugin-name.php

All of the class files also have Namespace defined. I’m not getting idea on how to include those in custom plugins so it would work like it was working in theme folder. Could we use autoloader in someway? Appreciate you comments and help

Thanks