Implement flake utility function mkAtomicFlake

[LiGoldragon]

A function to use atoms to create flake outputs.
The following code can be found in a test repo here:
https://github.com/LiGoldragon/aidiyz/blob/f25e895fb9aa32d8ae30fece60845db42fedd34a/flake.nix#L1

# flake.nix
{
  description = "An Atomic Flake";

  inputs.atom.url = "github:LiGoldragon/atom";

  outputs =
    inputs@{ self, atom, ... }:
    atom.mkAtomicFlake {
      manifest = ./. + "/simple@.toml";
      noSystemManifest = ./. + "/local-lib@.toml";
    };
}

# simple@.toml
[atom]
id = "simple"
version = "0.1.0"

[local.local-lib]
path = "local-lib@.toml"

[fetch.nixpkgs]
name = "nixpkgs"
import = true

[features]
default = ["system"]

[backend.nix]
fetcher = "npins"

# simple/mod.nix
let
  pkgs = use.nixpkgs {
    inherit system;
  };

  mkStringPackage =
    string:
    pkgs.runCommand string { } ''
      mkdir $out
      echo ${string} > $out/string.txt
    '';

in
{
  Packages.default = pkgs.hello;
  Packages.simpleString = mkStringPackage use.local-lib.simpleString;
  Packages.hashString = mkStringPackage use.local-lib.stringHash;

}

# local-lib@.toml
[atom]
id = "local-lib"
version = "0.1.0"

# local-lib/mod.nix
{
  SimpleString = "A simple string";
  StringHash = std.hashString "sha256" mod.simpleString;
}

[ic4-y]

I think this is going in a similar direction as #40 . Hence the same questions apply. It might not be the intended direction of atom-nix to support flakes more directly than necessary.

What do you think @nrdxp ?

[nrdxp]

I'm not against a flake utility function, in principle, as it might ease adoption, but it might be a bit before I can review this in depth. I have some pretty major overhauls to make in terms of atom-nix's internal API #48, so I don't necessarily want to lock any compatible mechanism down before that is a bit more stable.

There is also the question of whether something like this might better live in an small "flake" utility library atom.