LocalisationCache: Use file_get_contents instead of DOMDocument::load

DOMDocument::load fails to load the plurals data during the import
process.

This is a work-around for https://bugs.php.net/bug.php?id=64938. This
bug only rears its head when using Special:Import, because that is
essentially the only place in MediaWiki where we fiddle with
libxml_disable_entity_loader.

Bug: T58439
Change-Id: Idcb4ab1cef2a7b080543e7cc1cee5464fc476456