Skip Overwrite on extract in `Loader::element()`

Skip overwriting on `extract($args)` in `Loader::element()`

- Add `EXTR_SKIP` instead of default `EXTR_OVERWRITE`

http://www.concrete5.org/developers/bugs/5-6-1-2/loaderpackageelement-breaks-on-file-as-parameter/
https://github.com/concrete5/concrete5/pull/1067
https://github.com/concrete5/concrete5/pull/1033