Force re-cache of Canonical Namespaces

If an extension does something seemingly innocuous like calling
Title::getPrefixedText in a hook called before the end of
includes/Setup.php, it causes the list of canonical namespaces to be
cached early and breaks extensions that try to add namespaces from a
$wgExtensionFunctions callback.

This adds a call to recache the namespace list after calling those
$wgExtensionFunctions callbacks. Also clears the cache held by
the language objects.

Bug: 45031