如何让Drupal8访问web根目录之外的配置文件？

services:
  custom_module.config:
    class: Drupal\Core\Config\ConfigFactoryOverrideInterface
    tags:
      - { name: config.factory.override }
    arguments: ['@config.factory']

my_config_path: '../config/myconfig.yml'

<?php

namespace Drupal\custom_module\Form;

use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;

class CustomModuleSettingsForm extends ConfigFormBase {

  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return ['custom_module.settings'];
  }

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'custom_module_settings_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $config = $this->config('custom_module.settings');

    $form['my_config_path'] = [
      '#type' => 'textfield',
      '#title' => $this->t('My Config Path'),
      '#default_value' => $config->get('my_config_path'),
    ];

    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->config('custom_module.settings')
      ->set('my_config_path', $form_state->getValue('my_config_path'))
      ->save();

    parent::submitForm($form, $form_state);
  }

}

custom_module.settings_form:
  path: '/admin/config/custom_module/settings'
  defaults:
    _form: '\Drupal\custom_module\Form\CustomModuleSettingsForm'
    _title: 'Custom Module Settings'
  requirements:
    _permission: 'administer site configuration'