A generic hook_ctools_plugin_directory implementation.

hook_ctools_plugin_directory.php

<?php

/**
 * Implements hook_ctools_plugin_directory().
 */
function hook_ctools_plugin_directory($owner, $plugin_type) {

  $plugin_map = array(
    
    // Set the directory map as the key to the following 'owner' => 'plugin_type' combinations.
    "plugins/$plugin_type" => array(
      
      // Set owner => plugin_type so if they match, the directory above will be used.
      'panels' => 'styles',
      // Set to an array to allow more then one specific plugin types for this owner.
      'ctools' => array('content_types', 'cache'),

      // Set value to the variable so any value is accepted for this owner.
      'page_manager' => $plugin_type,
      
      // MORE...
      // 'stylizer' => $plugin_type,
      // 'panelizer' => $plugin_type,
    ),
    
    // Set other plugin paths, if needed. They might even contain the same owners as above,
    // if they are defined with a differente plugin type.
  );

  foreach ($plugin_map as $map => $plugins) {
    if (!is_array($plugins[$owner])) {
      $plugins[$owner] = array($plugins[$owner]);
    }
    if (isset($plugins[$owner]) && in_array($plugin_type, $plugins[$owner])) {
      return $map;
    }
  }
}