Don't use autodiscovery for core skins, move them to separate directories

Step one on the way to killing the autodiscovery mechanism and making
the core skins less intertwined with core.

This only moves the files and fixes hardcoded paths throughout core.
Any further changes will be done in separate patch(es).

Moved files:
* skins/MonoBook.php → skins/monobook/MonoBook.php
* skins/Vector.php   → skins/vector/Vector.php

Bug: 65748
Change-Id: Ib4bdda5ed3c133fce0113eb17fa39950aa812f87