OSShells, refactor shells to single option

nixosModules/apps/shells/app.nix

@@ -1,9 +1,29 @@
-{ ... }:
+{ config, pkgs, lib, ... }: let
+  fs = lib.fileset;
+  shellFilter = {name, ...}: name == "shell.nix";
+  shellImports = fs.toList (fs.fileFilter shellFilter ./.);
+  shellNames = map (
+    path: let
+      splitPath = lib.strings.splitString "/" path;
+      splitPathLen = builtins.length splitPath;
+    in builtins.elemAt splitPath (splitPathLen - 2)
+  ) shellImports;
+in {
+  imports = shellImports;
 
-{
-  imports = [
-    ./fish
-    ./xonsh
-    ./zsh
-  ];
+  options.shell = let 
+    shellNameEnum = lib.types.enum shellNames;
+  in {
+    defaultShell = lib.mkOption {
+      type = shellNameEnum;
+    };
+    enabledShells = lib.mkOption {
+      type = lib.types.listOf shellNameEnum;
+    };
+  };
+
+  config = {
+    users.defaultUserShell = pkgs."${config.shell.defaultShell}";
+    environment.shells = map (shell: pkgs."${shell}") config.shell.enabledShells;
+  };
 }