emacs-modes: replace melpa-generated with json format

This approach has several differences with emacs2nix:

- the updater uses a downloaded recipes.json and archive.json for commit information, it uses a local checkout only for hashing the recipes
- the generated file is JSON
- the updater is written in emacs lisp
- prefetch errors are put into an error key in the JSON, for review + meta.broken attributes are generated from it

The updater re-uses the existing generated file to memoize prefetched content-sha256s for commits, thus prefetching should normally be quite fast.