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

Skip overwiting on `extract($args)` in `Loader::element()`

- Add `EXTR_SKIP` instead of default `EXTR_OVERWRITE`
Scope and Check `Loader::element()` `$args`

Scope and check `Loader::element()` `$args` before `extract()`

- Change `$file` to `$_file`
- Change `$pkgHandle` to `$_pkgHandle`
- Throw exception if reserved argument names used
Merge pull request #1158 from mkly/skip_extract_overrite_loader_element

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