Clean up libraries validation code

Since @sasanikolic wrote the code for handling libraries, he would be the guy to talk to. The main issue was how the libraries module works currently.

Ok. I remember that @sasanikolic wrote a lot of code for handling one CSS and one JS file. This logic should be handled by libraries. Here is the code we are talking about:

image_widget_crop.module:

/**
 * Implements hook_library_info_alter().
 */
function image_widget_crop_library_info_alter(&$libraries, $extension) {
  if ($extension != 'image_widget_crop') {
    return;
  }

  $config = \Drupal::config('image_widget_crop.settings');
  if (!\Drupal::moduleHandler()
      ->moduleExists('libraries') && !$config->get('settings.library_url') && !$config->get('settings.css_url')
  ) {
    $libraries['cropper.integration']['js'] = [];
  }

  // Get the correct path of the Cropper js file (the user needs to manually
  // put the jquery.cropper.min.js in libraries/cropper folder or set the url
  // in the settings).
  if ($library_url = $config->get('settings.library_url')) {
    // Cloud hosted library, use external JavaScript.
    $libraries['cropper']['js'][$library_url] = [
      'type' => 'external',
      'minified' => TRUE,
    ];
  }
  elseif (\Drupal::moduleHandler()->moduleExists('libraries')) {
    $info = libraries_detect('cropper');
    $libraries['cropper'] += [
      'version' => $info['installed'] ? $info['version'] : 'web-hosted',
    ];
    if ($info['installed']) {
      // Because the library is self hosted, use files from library definition.
      if (!empty($info['files']['js'])) {
        foreach ($info['files']['js'] as $data => $option) {

          if (is_numeric($data)) {
            $option = "/{$info['library path']}/{$option}";
          }
          elseif (empty($option['type']) || $option['type'] == 'file') {
            $data = "/{$info['library path']}/{$data}";
          }

          $libraries['cropper']['js'][$data] = $option;
        }
      }
    }
  }

  // Add the local CSS to the libraries.
  if ($css_url = $config->get('settings.css_url')) {
    // Cloud hosted library, use external CSS.
    $libraries['cropper']['css']['component'][$css_url] = [
      'type' => 'external',
      'minified' => TRUE,
    ];
  }
  elseif (\Drupal::moduleHandler()->moduleExists('libraries')) {
    $info = libraries_detect('cropper');
    $libraries['cropper'] += [
      'version' => $info['installed'] ? $info['version'] : 'web-hosted',
    ];
    if ($info['installed']) {
      // Because the library is self hosted, use files from library definition.
      if (!empty($info['files']['css'])) {
        foreach ($info['files']['css'] as $data => $option) {

          if (is_numeric($data)) {
            $option = "/{$info['library path']}/{$option}";
          }
          elseif (empty($option['type']) || $option['type'] == 'file') {
            $data = "/{$info['library path']}/{$data}";
          }

          $libraries['cropper']['css']['theme'][$data] = $option;
        }
      }
    }
  }
}

src/Form/CropWidgetForm.php:

  /**
   * Validation for cropper library.
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm($form, $form_state);
    // TODO: Change the autogenerated stub.
    if (\Drupal::moduleHandler()->moduleExists('libraries')) {
      $directory = libraries_get_path('cropper') . '/dist/';
      $library = 'cropper.min.js';
      $css = 'cropper.min.css';
      if (!file_exists($directory . $library) || !file_exists($directory . $css)) {
        $form_state->setErrorByName('plugin', t('Either the library file or the CSS file is not present in the directory %directory.', array(
          '%directory' => '/' . $directory,
        )));
      }
    }
    else {
      if (empty($form_state->getValue('library_url')) || empty($form_state->getValue('css_url'))) {
        $form_state->setErrorByName('plugin', t('Either set the library and CSS locally and enable the libraries module or enter the remote URLs below. Check the README.md file for more information.'));
      }
      $cropper_cdn_url = 'https://cdnjs.com/libraries/cropper';
      if (!empty($form_state->getValue('library_url'))) {
        // Check if the name of the library in the remote URL is as expected.
        $library_url = $form_state->getValue('library_url');
        if (parse_url($library_url, PHP_URL_HOST) && parse_url($library_url, PHP_URL_PATH)) {
          $js = pathinfo($library_url, PATHINFO_BASENAME);
          if (!preg_match('/^cropper\.min\.js$/', $js)) {
            $form_state->setErrorByName('plugin', t('The naming of the library is unexpected. Double check that this is the real Cropper library. The URL for the minimized version of the library can be found on <a href="@url">Cropper CDN</a>.', ['@url' => $cropper_cdn_url]), 'warning');
          }
        }
        else {
          $form_state->setErrorByName('plugin', t('The remote URL for the library is unexpected. Please, provide the correct URL to the minimized version of the library found on <a href="@url">Cropper CDN</a>.', ['@url' => $cropper_cdn_url]), 'error');
        }
      }
      elseif (!empty($form_state->getValue('css_url'))) {
        // Check if the name of the library in the remote URL is as expected.
        $css_url = $form_state->getValue('css_url');
        if (parse_url($css_url, PHP_URL_HOST) && parse_url($css_url, PHP_URL_PATH)) {
          $css = pathinfo($css_url, PATHINFO_BASENAME);
          if (!preg_match('/^cropper\.min\.css$/', $css)) {
            $form_state->setErrorByName('plugin', t('The naming of the CSS is unexpected. Double check that this is the real Cropper CSS file. The URL for the minimized version of the CSS fuke can be found on <a href="@url">Cropper CDN</a>.', ['@url' => $cropper_cdn_url]), 'warning');
          }
        }
        else {
          $form_state->setErrorByName('plugin', t('The remote URL for the CSS file is unexpected. Please, provide the correct URL to the minimized version of the CSS file found on <a href="@url">Cropper CDN</a>.', ['@url' => $cropper_cdn_url]), 'error');
        }
      }
    }
  }