Initialize with packages in headers from package and tests

[psionic-k]

Within a well-formed elisp package, we know where all the headers are. If all requirments for the package and its tests are in headers, there's no need to update nix expressions manually after adding new package-requires information.

Probably I can implement this. Stop me if there's a better solution.

[akirak]

@psionic-k AFAIK this has been already done. See recipes-archive-melpa.json. There is deps field, and the file is automatically generated by the script in the directory. You can find other scripts in pkgs/applications/editors/emacs/elisp-packages in nixpkgs. Except for manual packages, it's automated.

[psionic-k]

@akirak I should have mentioned emacsWithPackagesFromUsePackage. It looks super close to what I need. I'm not sure.

Use case is for CI, so I need to discover the elisp files within the package repository.

The repo motivating this issue:
https://github.com/positron-solutions/elisp-repo-kit/tree/finish-first-release