feat(entrypoint): expose evalModules for external interop

e.g. colmena can make use of it to declare an heterogeneous set of
systems, mixing Liminix and NixOS systems.

Signed-off-by: Raito Bezarius <masterancpp@gmail.com>

default.nix

@@ -18,10 +18,12 @@ let
     };
   });
 
-  eval = pkgs.lib.evalModules {
+  evalModules = (import ./lib/eval-config.nix {
-    specialArgs = {
+    inherit nixpkgs pkgs;
-      modulesPath = builtins.toString ./modules;
+    inherit (pkgs) lib;
-    };
+  });
+
+  eval = evalModules {
     modules = [
       { _module.args = { inherit pkgs; inherit (pkgs) lim; }; }
       ./modules/hardware.nix
@@ -36,6 +38,7 @@ let
       ./modules/outputs.nix
     ];
   };
+
   config = eval.config;
 
   borderVm = ((import <nixpkgs/nixos/lib/eval-config.nix>) {
@@ -54,6 +57,8 @@ let
     ];
   }).config.system;
 in {
+  inherit evalModules;
+
   outputs = config.system.outputs // {
     default = config.system.outputs.${config.hardware.defaultOutput};
     optionsJson =

lib/eval-config.nix

@@ -0,0 +1,24 @@
+{ nixpkgs ? <nixpkgs>, pkgs ? (import <nixpkgs> {}), lib ? pkgs.lib }:
+args:
+let
+  modulesPath = builtins.toString ../modules;
+in
+# FIXME: we could replace the `lib` by a lib-only minimal nixpkgs.
+(import "${nixpkgs}/lib/modules.nix" { inherit lib; }).evalModules (args // {
+  specialArgs = (args.specialArgs or {}) // {
+    inherit modulesPath;
+  };
+  modules = (args.modules or []) ++ [
+    "${modulesPath}/hardware.nix"
+    "${modulesPath}/base.nix"
+    "${modulesPath}/busybox.nix"
+    "${modulesPath}/hostname.nix"
+    "${modulesPath}/kernel"
+    "${modulesPath}/s6"
+    "${modulesPath}/users.nix"
+    "${modulesPath}/outputs.nix"
+    # FIXME: we should let the caller inject `pkgs` and `lim` ideally...
+  ] ++ lib.optional (pkgs ? lim) {
+    _module.args = { inherit (pkgs) lim; };
+  };
+})