lib: init env-builders

This is not a part of the library set but it is just used to test how
bootstrappable they are when used both with non-flake- and flake-based
setups.

lib/env-builders.nix

@@ -0,0 +1,77 @@
+# This shouldn't be a part of the foodogsquared's library set. These are simply
+# environment builders used to build consistent environments throughout flake-
+# and non-flake-based Nix environments. Much of the things here are unofficial
+# and basically redoing what the upstream projects are doing so maintenance is
+# pretty much higher here.
+#
+# Despite named similarly to npins `sources` convention, it should also work
+# for flake-based setups as long as the inputs attrset from the flake output
+# function is the one that is passed.
+{ lib, foodogsquaredLib, sources }:
+
+let
+  nixosModules = ../modules/nixos;
+
+  # A set of nixos-generators modules including our custom ones.
+  nixosGeneratorModules =
+    let
+      officialFormats = builtins.readDir "${sources.nixos-generators}/formats";
+      unofficialFormats = builtins.readDir ../modules/nixos-generators;
+      formats = officialFormats // unofficialFormats;
+    in
+    lib.mapAttrs' (n: _: lib.nameValuePair (lib.removeSuffix ".nix" n) {
+      imports = [
+        "${sources.nixos-generators}/format-module.nix"
+        (
+          if (lib.hasAttr n officialFormats)
+          then "${sources.nixos-generators}/formats/${n}"
+          else "${../modules/nixos-generators}/${n}"
+        )
+      ];
+    }) formats;
+in
+rec {
+  mkNixosSystem = {
+    pkgs,
+    lib ? pkgs.lib,
+    system,
+    extraModules ? [ ],
+    specialArgs ? { },
+  }:
+    let
+      # Evaluating the system ourselves (which is trivial) instead of relying
+      # on nixpkgs.lib.nixosSystem flake output.
+      nixosSystem = args: import "${pkgs.path}/nixos/lib/eval-config.nix" args;
+    in
+    (lib.makeOverridable nixosSystem) {
+      inherit pkgs;
+      specialArgs = specialArgs // {
+        foodogsquaredUtils = import ./utils/nixos.nix { inherit lib; };
+        foodogsquaredModulesPath = builtins.toString nixosModules;
+      };
+      modules = extraModules ++ [{
+        nixpkgs.hostPlatform = lib.mkForce system;
+      }];
+
+      # Since we're setting it through nixpkgs.hostPlatform, we'll have to pass
+      # this as null.
+      system = null;
+    };
+
+  # A very very thin wrapper around `mkNixosSystem` to build with the given format.
+  mkNixosImage = {
+    pkgs,
+    system,
+    extraModules ? [ ],
+    specialArgs ? { },
+    format ? "iso",
+  }:
+    let
+      extraModules' = extraModules ++ [ nixosGeneratorModules.${format} ];
+      nixosSystem = mkNixosSystem {
+        inherit pkgs system specialArgs;
+        extraModules = extraModules';
+      };
+    in
+    nixosSystem.config.system.build.${nixosSystem.config.formatAttr};
+}