Incorrect handling of "file" on hook_ctools_plugin_api() implementations

Function ctools_plugin_api_include() makes incorrect use of the file value returned by hook_ctools_plugin_api(). Instead of assigned it to $file, it assigns the result of isset() to that variable.

This causes the parameter not to work, as ctools checks for existence of DRUPAL_ROOT . "/$plugin_info[path]/1", which never exists.

In addition, the module also recognizes undocumented key $api file, with the same error. It should probably be documented or removed.