Move core message dirs from $wgMessagesDirs to LocalisationCache::getMessagesDirs()

If $wgMessagesDirs is initially empty, we can optimize when batch loading
extensions:

    if ( !$wgMessagesDirs ) {
        $wgMessagesDirs = $cache['MessagesDirs'];
    }

With APC, this should be O(1) CPU time.

This was suggested by Tim in the code review of I7074b65d07c5.

Change-Id: I66fa907cdaafe18b74b5b9afaa8b6b1db069bea3