Follow updates in package-build

[akirak]

There will be a change (melpa/package-build#68) in package-build. Specifically, the recipe spec is under a change (e.g. accepting multiple subdirectory/exclude patterns) at melpa/package-build@0adc96b. I haven't found a concrete example for these changes yet, but I will have to modify this library to cover the new spec.

Unlike the other parties following those updates, this library is written in Nix, so it is impossible to simply copy-and-paste the "expand a file spec" function written in Emacs Lisp.