Use plugins instead of included files.

So here the refactored module that uses services.

But still, this is not really good implementation of moving variables implementation out of the hook.
I have some work in progress that will be moving each variable in the separate plugin.
So the implementation of variable will looks like that:

namespace Drupal\mgv\Plugin\GlobalVariable;

use Drupal\mgv\Plugin\GlobalVariableInterface;

/**
 * Class CurrentPath.
 *
 * @package Drupal\mgv\Plugin\GlobalVariable
 *
 * @Mgv(
 *   name = "current_path",
 * );
 */
class CurrentPath implements GlobalVariableInterface {

  /**
   * {@inheritdoc}
   */
  public function getValue(array $deps = []) {
    // Print the current path. This could be useful if you want to do a redirect
    // after a form is submitted, e.g. ?destination={{ current_path }}.
    return \Drupal::service('path.current')->getPath();
  }

}

Here a completed patch that moves variables into separate plugins.
So developers can simply define a variable using GlobalVariable plugin in own module and variable will appear in global_variables list.