Including third party libraries in plugin [closed]

The easiest thing would be to use some sort of dependency management. You could use git submodules (link to SCM page) or for e.g. Composer.

When you are using an autoloader (Composer ships with one) and have a PSR-4 (Fig standard) compliant autoloader (like Composer has), then you can just use \Namespace\SubNamespace and have everything available.