Make include_path functionality of Config and Translator opt-in

#4515 (which addresses #4443) and #4574 both introduce features that use PHP's `include_path` to identify application assets. While the functionality has reasonable use cases, as implemented, it introduces some performance impact, as the components look on the `include_path` for each and every file. Additionally, if `.` is not on the `include_path`, files that resolve from the current working directory will not resolve.