Yet another idea. Supporting wildcards in the alias like this:

nixDirAliases = {
  nixosModules = [ "nixos/modules/*"]
};

So you could support the layout:

The right side can be a path, yes; problem is the left side which doesn't support nesting.
I was considering having it recurse, but even then not sure if that would work since the key is not a concrete option.

for that you could write (assuming the dirs are dirs you want to load as attrs):

devShells = {
  ansible = flakelight.importDir "ansible/nix";
  autotest = flakelight.importDir "autotest/nix";
  dbusmock = flakelight.importDir "dbus-mock/nix";
};

or as I'd more likely write:

devShells = lib.genAttrs [ "ansible" "autotest" "dbus-mock" ] (n: flakelight.importDir "${n}/nix";

The right side can be a path, yes; problem is the left side which doesn't support nesting.
I was considering having it recurse, but even then not sure if that would work since the key is not a concrete option.

for that you could write (assuming the dirs are dirs you want to load as attrs):

devShells = {
  ansible = flakelight.importDir "ansible/nix";
  autotest = flakelight.importDir "autotest/nix";
  dbusmock = flakelight.importDir "dbus-mock/nix";
};

or as I'd more likely write:

devShells = lib.genAttrs [ "ansible" "autotest" "dbus-mock" ] (n: flakelight.importDir "${n}/nix");

The right side can be a path, yes; problem is the left side which doesn't support nesting.
I was considering having it recurse, but even then not sure if that would work since the key is not a concrete option.

for that you could write (assuming the dirs are dirs you want to load as attrs):

devShells = {
  ansible = flakelight.importDir "ansible/nix";
  autotest = flakelight.importDir "autotest/nix";
  dbusmock = flakelight.importDir "dbus-mock/nix";
};

or as I'd more likely write:

devShells = lib.genAttrs
  [ "ansible" "autotest" "dbus-mock" ]
  (n: flakelight.importDir "${n}/nix");

Yeah, for this usecase, I'd recommend putting the namespace in the file name rather than dir since with dirs its different in the filesystem and the nix code. nix flake show or nix eval .#nixosModules --apply __attrNames would show all of them without namespacing. IMO making it .#nixosModules.desktops-gnome.nix would be more descriptive.

though if you want that layout, i'd set nixDirAliases.nixosModules = [ "nixos/modules" ] and then have nixos/modules/default.nix be:

{flakelight, lib, ...}:
lib.pipe ./. [
  builtins.readDir
  (lib.filterAttrs (_: v: v == "directory")
  lib.attrNames
  (map (d: flakelight.importDir (./. + d)))
  (lib.foldr (a: b: a // b) {})
] 

I'm not sure how the glob would match in general, and what to do about matching files etc. I think this case would be good to specify how it should be loaded.

I guess the idea is just to use directories to better organize modules.

Your suggestion with using default.nix is the exact thing I want to minimize and ideally avoid.
It is hard to read for users without any Nix experience (I really try to keep the Nix files simple, so they are primarily declarative), and again it is boilerplate, that ideally it could be fully avoided with e.g. the wildcard feature.

I guess the idea is just to use directories to better organize modules.

Your suggestion with using default.nix is the thing I want to minimize and ideally avoid.
It is hard to read for users without any Nix experience (I really try to keep the Nix files simple, so they are primarily declarative), and again it is boilerplate, that ideally it could be fully avoided with e.g. the wildcard feature.

I guess the developer's idea is to use the directories to categorize modules.

Your suggestion with using default.nix is the thing I want to minimize and ideally avoid.
It is hard to read for users without any Nix experience (I really try to keep the Nix files simple, so they are primarily declarative), and again it is boilerplate, that ideally it could be fully avoided with e.g. the wildcard feature.

I guess the developer's idea is to use the directories to categorize modules.

Your suggestion with using default.nix is the thing I want to minimize and ideally avoid.
It is hard to read for users without any Nix experience (I really try to keep the Nix files simple, so they are primarily declarative), and again it is boilerplate, that ideally could be avoided with e.g. the wildcard feature.

maybe it would work to have a common nix lib provide a lib glob function?

though using nixDirAliases.nixosModules = lib.pipe ./nixos/modules [ builtins.readDir lib.attrNames (map (d: "nixos/modules/${d}"))] would be less code than pulling in a lib if thats all thats in it

How can you access lib here?:

nixDirAliases = {
  nixosModules = lib.subdirs "nixos/modules"
};