emacs-packages: Clean up redundant packages & allow overriding attrsets

[adisbladis]

Motivation for this change

I'm creating an auto updating emacs overlay based on https://github.com/adisbladis/exwm-overlay which will contain all our auto-generated emacs package sets and need the ability to override the loading of these.

Things done

With these changes I can generate a new melpaPackages from my locally generated json like this:

myMelpaPackages = emacsPackagesNg.melpaPackages.override {
  archiveJson = ./repos/recipes-archive-melpa.json;
};

And then use it to create a new emacsPackagesNg set:

emacsPackagesNg.override { melpaPackages = myMelpaPackages; }

or for a more complete example see the melpa branch in my overlay repo: nix-community/emacs-overlay@f7edeaa

Also the updater script can now be called from anywhere and the output JSON will be in current working directory.

While i do think this method is a bit crude it was by far the easiest way I could come up with to achieve what I wanted.

I also started cleaning up a bit of redundant packages that come autogenerated from melpa that we don't need explicit derivations for.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @gridaphobe @oxij @talyz @mdorman

edit: The scope of this PR grew a bit as I went along and cleaned up redundant packages. If need be I can split it into 2 PRs.

If you'd like to join in on this effort or if you're just interested in the emacs packaging in nixpkgs I've created #nixos-emacs on freenode.

[adisbladis]

I will merge these changes within the next 24 hours (probably way sooner) if there are no objections.

[multun]

@adisbladis 978b346 broke irony :(
Do you have any idea why ?