Ensure package directories for source packages from checkouts

* lisp/emacs-lisp/package-vc.el (package-vc-install-from-checkout):
Set the :dir entry, since `package-vc--unpack-1' assumes the field is
set, as is the case when invoking `package-vc--unpack'.

lisp/emacs-lisp/package-vc.el

@@ -783,6 +783,7 @@ name from the base name of DIR."
     (package-vc--unpack-1
      (package-desc-create
       :name (intern name)
+      :dir pkg-dir
       :kind 'vc)
      (file-name-as-directory pkg-dir))))