Use a 3rd part library when uninstalling a module
Not knowing what "operations" you are performing with this composer lib, I suggest you might want to just think of when you actually want to perform these operations. Doing it on Uninstall seems like a bad idea. Why not utilize hook_disable, instead of hook_uninstall, which should allow you to do things just before the module gets disabled.

If you really want to do it in a hook_uninstall, then you can load your .module file using drupal_load() and call the functions in your .module file from hook_uninstall.