nixos/system/build: Extract

Modules that do not depend on e.g. toplevel should not have to include it just to set
things in `system.build`. As a general rule, this keeps tests simple, usage flexible
and evaluation fast. While one module is insignificant, consistency and good practices
are.

nixos/modules/system/activation/top-level.nix

@@ -139,21 +139,13 @@ in
 
 {
   imports = [
+    ../build.nix
     (mkRemovedOptionModule [ "nesting" "clone" ] "Use `specialisation.«name» = { inheritParentConfig = true; configuration = { ... }; }` instead.")
     (mkRemovedOptionModule [ "nesting" "children" ] "Use `specialisation.«name».configuration = { ... }` instead.")
   ];
 
   options = {
 
-    system.build = mkOption {
-      internal = true;
-      default = {};
-      type = with types; lazyAttrsOf (uniq unspecified);
-      description = ''
-        Attribute set of derivations used to setup the system.
-      '';
-    };
-
     specialisation = mkOption {
       default = {};
       example = lib.literalExpression "{ fewJobsManyCores.configuration = { nix.buildCores = 0; nix.maxJobs = 1; }; }";

nixos/modules/system/build.nix

@@ -0,0 +1,18 @@
+{ lib, ... }:
+let
+  inherit (lib) mkOption types;
+in
+{
+  options = {
+
+    system.build = mkOption {
+      internal = true;
+      default = {};
+      type = with types; lazyAttrsOf (uniq unspecified);
+      description = ''
+        Attribute set of derivations used to set up the system.
+      '';
+    };
+
+  };
+}