Allow for autoloading modules

Autoloads files within modules/ and makes them available by their module
type and name, e.g. `mixins-zsh`, `editor-nvim` and `users-opdavies`
when imported with `inputs.self.nixosModules`.

Based on afab322e6d/modules/default.nix.

This assumes there will be a modules/*.nix file and currently doesn't
work with modules/*/default.nix. This is something I'd like to add in
the future.

I also want find a cleaner way to pass arguments into these shortcuts as
this also doesn't work in their implementation as far as I can see.

modules/default.nix

@@ -0,0 +1,57 @@
+# Based on https://github.com/MatthewCroughan/nixcfg/blob/afab322e6da20cc038d8577dd4a365673702d183/modules/default.nix.
+# Autoloads files within modules/ and makes them available by their module
+# type and name, e.g. `mixins-zsh`, `editor-nvim` and `users-opdavies` when
+# imported with `inputs.self.nixosModules`.
+
+{ lib }:
+
+let
+  inherit (builtins)
+    attrNames
+    filter
+    listToAttrs
+    map
+    readDir
+    toString
+    ;
+
+  inherit (lib)
+    filterAttrs
+    foldAttrs
+    hasSuffix
+    mapAttrsToList
+    nameValuePair
+    removeSuffix
+    zipListsWith
+    ;
+
+  inherit (lib.filesystem) listFilesRecursive;
+
+  generateModules =
+    folder: prefix:
+
+    let
+      findSuffix = suffix: dir: (filter (x: (hasSuffix suffix (toString x))) (listFilesRecursive dir));
+
+      allNixFiles = findSuffix ".nix" folder;
+
+      allModuleNames = map (removeSuffix ".nix") (map baseNameOf allNixFiles);
+
+      zippedList = (
+        zipListsWith (x: y: nameValuePair (prefix + "-" + x) (import y)) allModuleNames allNixFiles
+      );
+    in
+    listToAttrs zippedList;
+  generateModulesAuto =
+    root:
+    let
+      moduleFolderNames = attrNames (filterAttrs (n: v: v == "directory") (readDir (toString root)));
+      moduleFolderPaths = map (x: (toString root) + "/" + x) moduleFolderNames;
+
+      zippedList = listToAttrs (
+        zipListsWith (x: y: nameValuePair x y) moduleFolderNames moduleFolderPaths
+      );
+    in
+    foldAttrs (item: acc: item) { } (mapAttrsToList (n: v: generateModules v n) zippedList);
+in
+generateModulesAuto ./.