nixpkgs module: Fix defaulting of localSystem and system

Take two of #40708 (4fe2898608).

That PR attempted to bidirectionally default `config.nixpkgs.system` and
`config.nixpkgs.localSystem.system` to each be updated by the other. But
this is not possible with the way the module system works. Divergence in
certain cases in inevitable.

This PR is more conservative and just has `system` default `localSystem`
and `localSystem` make the final call as-is. This solves a number of
issues.

 - `localSystem` completely overrides `system`, just like with nixpkgs
 proper. There is no need to specify `localSystem.system` to clobber the
 old system.

 - `config.nixpkgs.localSystem` is exactly what is passed to nixpkgs. No
 spooky steps.

 - `config.nixpkgs.localSystem` is elaborated just as nixpkgs would so
 that all attributes are available, not just the ones the user
 specified.

The remaining issue is just that `config.nixpkgs.system` doesn't update
based on `config.nixpkgs.localSystem.system`. It should never be
referred to lest it is a bogus stale value because
`config.nixpkgs.localSystem` overwrites it.

Fixes #46320

nixos/modules/misc/nixpkgs.nix

@@ -62,12 +62,11 @@ in
     pkgs = mkOption {
       defaultText = literalExample
         ''import "''${nixos}/.." {
-            inherit (config.nixpkgs) config overlays localSystem crossSystem;
+            inherit (cfg) config overlays localSystem crossSystem;
           }
         '';
       default = import ../../.. {
-        localSystem = { inherit (cfg) system; } // cfg.localSystem;
-        inherit (cfg) config overlays crossSystem;
+        inherit (cfg) config overlays localSystem crossSystem;
       };
       type = pkgsType;
       example = literalExample ''import <nixpkgs> {}'';
@@ -140,8 +139,11 @@ in
 
     localSystem = mkOption {
       type = types.attrs; # TODO utilize lib.systems.parsedPlatform
-      default = { system = builtins.currentSystem; };
+      default = { inherit (cfg) system; };
       example = { system = "aarch64-linux"; config = "aarch64-unknown-linux-gnu"; };
+      # Make sure that the final value has all fields for sake of other modules
+      # referring to this. TODO make `lib.systems` itself use the module system.
+      apply = lib.systems.elaborate;
       defaultText = literalExample
         ''(import "''${nixos}/../lib").lib.systems.examples.aarch64-multiplatform'';
       description = ''
@@ -180,6 +182,7 @@ in
     system = mkOption {
       type = types.str;
       example = "i686-linux";
+      default = { system = builtins.currentSystem; };
       description = ''
         Specifies the Nix platform type on which NixOS should be built.
         It is better to specify <code>nixpkgs.localSystem</code> instead.
@@ -196,6 +199,7 @@ in
         </programlisting>
         See <code>nixpkgs.localSystem</code> for more information.
 
+        Ignored when <code>nixpkgs.localSystem</code> is set.
         Ignored when <code>nixpkgs.pkgs</code> is set.
       '';
     };