title: Wordpress: activate_plugin function doesn't find the file
body: <p>I'm trying to activate a plugin from the theme's functions.php, however I get the error 'Plugin file does not exist' even though the path is set correctly. 
  Here's my code:</p>
  
  <pre><code>function activate_plugins( $plugin_path ) {
  
      // Get already-active plugins   
      $active_plugins = get_option('active_plugins');
      // Make sure your plugin isn't active
      if (isset($active_plugins[$plugin_path]))
          return;
  
      // Include the plugin.php file so you have access to the activate_plugin() function
  require_once(ABSPATH .'/wp-admin/includes/plugin.php');
      // Activate your plugin
      $res = activate_plugin($plugin_path);
      if ( is_wp_error( $res ) ) {
          echo $res-&gt;get_error_message();
      }
  
      return null;
  }
  activate_plugins( get_bloginfo('template_directory') . '/plugins/multiple-post-thumbnails/multi-post-thumbnails.php' );
  </code></pre>
  
  <p>I am trying to load the plugin from the theme folder because I'm currently developing a theme which I want to sell in the future, and don't want the customer to have to install the plugin manually after installing the theme. What can I do?</p>