[5.0] Normalize view names with namespaces with slashes properly

Had to move `const HINT_PATH_DELIMITER` to the `ViewFinderInterface`. `static::HINT_PATH_DELIMITER` in classes implementing the interface still works. http://3v4l.org/Z616Z

This does prevent end users from defining their own namespace delimiter. We could make a method `ViewFinderInterface->getDelimiter()` method instead. Thoughts?

Ref https://github.com/laravel/framework/pull/5756#commitcomment-7830096