Defer package loading by default

This is what we have done explicitly all along, so we also can make it the
default (I didn't know this was possible until right now …).

init.el

@@ -40,7 +40,8 @@
   (package-initialize))
 
 (eval-when-compile
-  (setq use-package-enable-imenu-support t)
+  (setq use-package-enable-imenu-support t
+        use-package-always-defer t)
 
   (dolist (package '(bind-key use-package))
     (unless (package-installed-p package)