Support other package sets

[infinisil]

We want to use the pkgs/by-name convention for other package sets, e.g. NixOS/nixpkgs#299347.

Here's a rough idea how this could look like:

Package set config

Define some Nix config file to determine which package sets exist:

# pkgs/top-level/by-name-sets-config.nix
{
  packageSets = {
    toplevel = {
      attrPath = [ ];
      path = ./pkgs/by-name;
      shardLength = 2;
    };
    mySet = {
      # Any attrpath that will contain the attributes
      attrPath = [ "mySet" ];
      path = ./pkgs/mySet/by-name;
      # No sharding
      shardLength = null;
    };
  };
}

Read the packages in Nix

Create a Nix file to automatically call all of them like the current by-name-overlay.nix:

# pkgs/top-level/by-name-sets.nix
{
  toplevel = callPackage: {
    hello = <...>;
  };
  mySet = callPackage: {
    hi = <...>;
  };
}

For each package set declared, define it by importing by-name-sets.nix, like

mySet = lib.makeScope pkgs.newScope (self:
  (import ./pkgs/top-level/by-name-sets.nix).mySet self.callPackage
) 

Check the packages with the tool

Adjust this tool to read a by-name-sets-config.nix file for determining what it needs to check and how.