Trying to use Containers in a WP way

test.php

<?php
/**
 * Main Plugin
 * @version  1.0.0
 */
namespace Backcourt;

use Backcourt\Vendor\League\Container\Container as Container;

defined( 'ABSPATH' ) || exit;
/**
 * Main plugin class.
 */
final class Plugin {

	/**
     * Plugin version.
     *
     * @var string
     */
    const VERSION = '1.0.0';

	/**
	 * Holds an instance of the class.
	 *
     * @var Plugin
     */
    private static $instance;

	/**
	 * Holds an instance of the Container.
	 *
     * @var Container
     */
    private $container;
  /**
	 * The list of Admin classes to register.
	 *
	 * @var string[]
	 */
	  private $service_providers = array(
		'Admin\Products\Metabox',
    		'Admin\Products\Input'
	);
  
	/**
	 * Constructor.
	 */
	private function __construct() {
		// Initialize dependency injection.
		$this->container = new Container();
    
    		foreach ( $this->service_providers as $service_provider_class ) {
      			// There may be occasions where you wish the service to be 
      			// the same instance each time you retrieve it. There are two ways to achieve this, declare it as shared
			$this->container->share( $service_provider_class );
		}
	}

	/**
	 * init.
	 */
	public function init() {
		$this->container->get('Admin\Products\Metabox')->init();
	}

	/**
	 * Shortcut to get a class from the container.
	 */
	public function get( $class ) {
		return $this->container->get( $class );
	}

	/**
	 * Gets the main instance.
	 *
	 * Ensures only one instance can be loaded.
	 *
	 * @return \Backcourt\Plugin
	 */
	public static function get_instance() {

		if ( null === self::$instance ) {
			self::$instance = new self();
		}

		return self::$instance;
	}

}

Test_With_Method_injector.php

<?php
/**
 * Main Plugin
 * @version  1.0.0
 */
namespace Backcourt;

use Backcourt\Vendor\League\Container\Container as Container;
use Backcourt\Vendor\League\Container\Definition\Definition as BaseDefinition;

defined( 'ABSPATH' ) || exit;
/**
 * Main plugin class.
 */
final class Plugin extends BaseDefinition {

	/**
     * Plugin version.
     *
     * @var string
     */
    const VERSION = '1.0.0';

	/**
	 * Holds an instance of the class.
	 *
     * @var Plugin
     */
    private static $instance;

	/**
	 * Holds an instance of the Container.
	 *
     * @var Container
     */
    private $container;
	/**
	 * The standard method that we use for dependency injection.
	 */
	public const ATTACH_HOOKS = 'init';

	/**
	 * The list of Admin classes to register.
	 *
	 * @var string['class_name'=>'method']
	 */
	  private $service_providers = array(
		'Admin\Products\Metabox'=>'init',
    		'Admin\Products\Input'=>'init',
	);
  
	/**
	 * Constructor.
	 */
	private function __construct() {
		// Initialize dependency injection.
		$this->container = new Container();
    
    		foreach ( $this->service_providers as $service_provider_class => $method ) {
      			// There may be occasions where you wish the service to be 
      			// the same instance each time you retrieve it. There are two ways to achieve this, declare it as shared
			// $this->container->share( $service_provider_class );
			$this->resolveClass( $service_provider_class );
		}
	}

	/**
	 * init.
	 */
	public function init() {
		//$this->container->get('Admin\Products\Metabox')->init();
	}

	/**
	 * Shortcut to get a class from the container.
	 */
	public function get( $class ) {
		return $this->container->get( $class );
	}
	
	/**
	 * Resolve a class using method injection instead of constructor injection.
	 *
	 * @param string $concrete The concrete to instantiate.
	 *
	 * @return object
	 */
	protected function resolveClass( string $concrete ) {
		$resolved = $this->resolveArguments( $this->arguments );
		$concrete = new $concrete();

		// Constructor injection causes backwards compatibility problems
		// so we will rely on method injection via an internal method.
		if ( method_exists( $concrete, static::ATTACH_HOOKS ) ) {
			call_user_func_array( array( $concrete, static::ATTACH_HOOKS ), $resolved );
		}

		return $concrete;
	}

	/**
	 * Gets the main instance.
	 *
	 * Ensures only one instance can be loaded.
	 *
	 * @return \Backcourt\Plugin
	 */
	public static function get_instance() {

		if ( null === self::$instance ) {
			self::$instance = new self();
		}

		return self::$instance;
	}

}