wrappers: expose platform wrapper modules via build.*Module options

Expose the platform wrapper modules as the Nixvim configuration options
`build.nixosModule`, `build.homeModule`, and `build.nixDarwinModule`.
This makes it possible to reuse a single Nixvim configuration across
NixOS, Home Manager, and nix-darwin without re-importing modules into
`programs.nixvim` manually.

Evaluating these wrapper modules requires a "bare" Nixvim configuration;
one that does not define `pkgs` or `nixpkgs.hostPlatform`. Such a
configuration would normally fail to evaluate, but disabling
`_module.check` provides a sufficiently lazy evaluation to access the
wrapper options.

To prevent the `_module.check = false` module from leaking into user
configs, it has a unique module key and gets disabled inside the wrapper
modules (`wrappers/_shared.nix`).

wrappers/nixos.nix

@@ -1,4 +1,7 @@
-self:
+{
+  self,
+  extendModules,
+}:
 {
   config,
   lib,
@@ -9,21 +12,20 @@ let
     mkIf
     ;
   cfg = config.programs.nixvim;
-  evalArgs = {
-    extraSpecialArgs = {
-      nixosConfig = config;
-    };
-    modules = [
-      ./modules/nixos.nix
-    ];
-  };
 in
 {
   _file = ./nixos.nix;
 
   imports = [
     (import ./_shared.nix {
-      inherit self evalArgs;
+      inherit self;
+      inherit
+        (extendModules {
+          specialArgs.nixosConfig = config;
+          modules = [ ./modules/nixos.nix ];
+        })
+        extendModules
+        ;
       filesOpt = [
         "environment"
         "etc"