registration: Initialize PSR-4 namespaces at same stage as normal autoloader

registration: Initialize PSR-4 namespaces at same stage as normal autoloader

readFromQueue() injects the content of AutoloadClasses to
$wgAutoloadClasses however it missed doing the same for
AutoloadNamespaces.

When using the installer with an extension having AutoloadNamespaces
set, its classes would not be found.

Make ExtensionRegistry append to AutoLoader::$psr4Namespaces, and add
a test to cover the new behavior.

Bug: T195783
Change-Id: Id61155867a4ca7d9bc4a347f8671da74b0fa490b
(cherry picked from commit 224864ebddab2e448f9ae4f247b85c652d9df42e)