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.

Bug: 45031
GitHub: https://github.com/wikimedia/mediawiki-core/pull/1
Change-Id: Ie00cbcd8ca0147663224eff6a6fe73fa70e1bbb4
(cherry picked from commit 9c806d1c3a6cd53e819a8088f3f47f2ed3a3f721)