[PHP7] Add support for package private class, trait and interface

Consumes PR #931 

TBD together with RFC
It might break if internally it uses case-insensitive to differentiate class names. However, only people who really know how PHP would consider problems. For instance, some filesystems, e.g., EXT4, allow files named ``User.php``, ``uSer.php`` and ``user.php`` respectively to exist in the same folder. How would developers implement autoloader to pick? Which one is the chosen one?