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>

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; };
+  };
+})