Adding a Spacemacs layer for “classic” emacs mode

Question

I' trying to wrap https://github.com/hading/chordpro-mode in a layer for Spacemacs. I ran M-x configuration-layer/create-layer and from there followed the documentation: https://www.spacemacs.org/doc/LAYERS.html.

Now I have a file ~/.emacs.d/private/chordpro-mode/packages.el, containing this (without the doc-comments):

(defconst chordpro-mode-packages
  '(
    chordpro
    (chordpro :location (recipe
                             :fetcher github
                             :repo "https://github.com/hading/chordpro-mode"))
    )

(use-package chordpro
            :defer t
            :init
            :mode "\\.pro$"
            )

I also added chordpro-mode to dotspacemacs-configuration-layers in .spacemacs. Now, starting Emacs it gives me the bad stare: Symbol's function definition is void: use-package. The error goes away if I remove the latter entry, but still Spacemacs does not load the mode when I open a *.pro file and instead gives me File mode specification error: (file-missing Cannot open load file chordpro-mode)

I don't know what I did wrong. Can someone please point me to where I'm off?