Find licenses in a bunch of nix sets.

license.nix

    licensesIn = acc: p: {
      res = if builtins.isAttrs p && builtins.hasAttr "meta" p then
        pkgs.lib.unique (acc.res ++ [{
          package = p.pname or p.name or "N/A";
          version = p.version or "N/A";
          url = p.meta.homepage or "N/A";
          licenses = (if builtins.isList (p.meta.license or "oops") then
            map (l: l.spdxId) p.meta.license
          else
            [ p.meta.license.spdxId or "N/A" ]);
        }])
      else
        builtins.trace p acc.res;
      subs = pkgs.lib.unique (acc.subs ++ ((p.buildInputs or [ ])
        ++ (p.nativeBuildInputs or [ ]) ++ (p.propagatedBuildInputs or [ ])));
    };

    grouniou = { res, subs }:
      if pkgs.lib.traceSeq ([ (builtins.length subs) (builtins.length res) ])
      (builtins.length subs) == 0 then
        res
      else
        grouniou (builtins.foldl' licensesIn {
          res = res;
          subs = [ ];
        } subs);

    licenses = grouniou {
      res = [ ];
      subs = roots;
    };

    li = pkgs.writeTextFile "crevette" (builtins.toJSON licenses);