feat(flake)!: convert to fs-defined modules

Place all modules in either:
    - `modules/${manager}/${module}.nix`
    - `modules/${manager}/${module}/default.nix`

- `${manager}` - One of `nixos`, `nix-on-droid`, `nix-darwin` or `home-manager`
- `${module}` - Module name (autoimported by configurations, used for exporting)

modules/home-manager/README.md

@@ -0,0 +1,5 @@
+Add your reusable home-manager modules to this directory (<https://nixos.wiki/wiki/Module>)
+
+These are considered modules:
+- files, ending in `.nix` (excluding `default.nix`)
+- directories, containing `default.nix`

modules/nix-darwin/README.md

@@ -0,0 +1,5 @@
+Add your reusable nix-darwin modules to this directory (<https://nixos.wiki/wiki/Module>)
+
+These are considered modules:
+- files, ending in `.nix` (excluding `default.nix`)
+- directories, containing `default.nix`

modules/nix-on-droid/README.md

@@ -0,0 +1,5 @@
+Add your reusable nix-on-droid modules to this directory (<https://nixos.wiki/wiki/Module>)
+
+These are considered modules:
+- files, ending in `.nix` (excluding `default.nix`)
+- directories, containing `default.nix`

modules/nixos/README.md

@@ -0,0 +1,5 @@
+Add your reusable NixOS modules to this directory (<https://nixos.wiki/wiki/Module>)
+
+These are considered modules:
+- files, ending in `.nix` (excluding `default.nix`)
+- directories, containing `default.nix`

util/default.nix

@@ -37,7 +37,10 @@ rec {
           lib.isAttrs (builtins.getAttr directory attrs))
         directories);
 
+  # Boolean helpers
   and = lib.all lib.id;
+  or  = lib.any lib.id;
+  eq  = x: y: x == y;
 
   # pkgs helpers
   forEachSystem = lib.genAttrs [
@@ -53,18 +56,68 @@ rec {
       (system:
         f nixpkgs.legacyPackages.${system});
 
+  # Modules helpers
+  createModules = baseDir: { passthru ? { inherit inputs outputs; }, ... }:
+    lib.pipe baseDir [
+      # Read given directory
+      builtins.readDir
+      # Map each entry to a module
+      (lib.mapAttrs'
+        (name: type:
+          let
+            moduleDir = baseDir + "/${name}";
+          in
+          if and [
+            (type == "directory")
+            (hasFiles ["default.nix"] (builtins.readDir moduleDir))
+          ] then
+            # Classic module in a directory
+            lib.nameValuePair
+              name
+              (import moduleDir)
+          else if and [
+            (type == "regular")
+            (lib.hasSuffix ".nix" name)
+          ] then
+            # Classic module in a file
+            lib.nameValuePair
+              (lib.removeSuffix ".nix" name)
+              (import moduleDir)
+          else
+            # Invalid module
+            lib.nameValuePair
+              name
+              null))
+      # Filter invalid modules
+      (lib.filterAttrs
+        (moduleName: module:
+          module != null))
+      # Passthru if needed
+      (lib.mapAttrs
+        (moduleName: module:
+          if and [
+            (builtins.isFunction
+              module)
+            (eq
+              (lib.pipe module [builtins.functionArgs builtins.attrNames])
+              (lib.pipe passthru [builtins.attrNames]))
+          ]
+          then module passthru
+          else module))
+    ];
+
   # Modules
-  nixosModules = import ../modules/nixos;
-  nixOnDroidModules = import ../modules/nix-on-droid;
-  nixDarwinModules = import ../modules/nix-darwin;
-  homeManagerModules = import ../modules/home-manager;
+  nixosModules       = createModules ../modules/nixos        { };
+  nixOnDroidModules  = createModules ../modules/nix-on-droid { };
+  nixDarwinModules   = createModules ../modules/nix-darwin   { };
+  homeManagerModules = createModules ../modules/home-manager { };
 
   # Machines
   machines = recurseDir ../machines;
   homeManagerMachines = machines.home-manager or { };
-  nixDarwinMachines = machines.nix-darwin or { };
-  nixOnDroidMachines = machines.nix-on-droid or { };
-  nixosMachines = machines.nixos or { };
+  nixDarwinMachines   = machines.nix-darwin   or { };
+  nixOnDroidMachines  = machines.nix-on-droid or { };
+  nixosMachines       = machines.nixos        or { };
 
   # Configuration helpers
   mkNixosHost = root: system: hostname: users: lib.nixosSystem {