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
