Allow engines to live in lib/

Now release_info engine is implemented in lib/Drush/UpdateService/ReleaseInfo.php. Other engines are considered legacy (while living in a command subfolder).

Also cleaned up and reduced some unneeded complexity.

API Changes:

```diff
-function drush_get_user_selected_engine(&$config, $engine_info)
-function drush_find_engine_to_use(&$config, $engine_info, $selected_engine = NULL)
+function drush_select_engine($config, $engine_info, $preferred = NULL);

-function drush_load_engine($type, $engine, $version = NULL, $path = NULL, $engine_config = NULL)
+function drush_load_engine($type, $engine, $config = NULL)

-function drush_include_engine($type, $selected_engine, $version = NULL, $path = NULL, $engine_config = NULL)
+function drush_include_engine($type, $engine, $config = NULL)
```