refactor: split flake.nix into multiple files

nix/win32-package.nix

@@ -0,0 +1,69 @@
+{ pkgs, naersk, fenixPkgs, system, version }:
+packageName:
+let
+  pkgsWin32 = pkgs.pkgsCross.mingw32;
+  rustTarget = pkgsWin32.stdenv.hostPlatform.rust.rustcTarget;
+  toolchainWin = fenixPkgs.combine [
+    fenixPkgs.stable.rustc
+    fenixPkgs.stable.cargo
+    fenixPkgs.targets.${rustTarget}.stable.rust-std
+  ];
+  naerskWin = pkgs.callPackage naersk {
+    cargo = toolchainWin;
+    rustc = toolchainWin;
+  };
+
+  # Get rid of MCF Gthread library.
+  # See <https://github.com/NixOS/nixpkgs/issues/156343>
+  # and <https://discourse.nixos.org/t/statically-linked-mingw-binaries/38395>
+  # for details.
+  #
+  # Use DWARF-2 instead of SJLJ for exception handling.
+  winCC = pkgsWin32.buildPackages.wrapCC (
+    (pkgsWin32.buildPackages.gcc-unwrapped.override
+      ({
+        threadsCross = {
+          model = "win32";
+          package = null;
+        };
+      })).overrideAttrs (oldAttr: {
+      configureFlags = oldAttr.configureFlags ++ [
+        "--disable-sjlj-exceptions"
+        "--with-dwarf2"
+      ];
+    })
+  );
+  targetCc = "${winCC}/bin/${winCC.targetPrefix}cc";
+in
+naerskWin.buildPackage {
+  pname = packageName;
+  cargoBuildOptions = x: x ++ [ "--package" packageName ];
+  inherit version;
+  strictDeps = true;
+  src = pkgs.lib.cleanSource ../.;
+  nativeBuildInputs = [
+    pkgs.perl # Needed to build vendored OpenSSL.
+    pkgs.nasm # aws-lc-sys requires it
+  ];
+  depsBuildBuild = [
+    winCC
+  ];
+  buildInputs = [
+    pkgsWin32.windows.pthreads
+  ];
+  auditable = false; # Avoid cargo-auditable failures.
+  doCheck = false; # Disable test as it requires network access.
+
+  CARGO_BUILD_TARGET = rustTarget;
+  TARGET_CC = "${targetCc}";
+  CFLAGS_i686_pc_windows_gnu = "-I${pkgsWin32.windows.pthreads}/include";
+  CARGO_BUILD_RUSTFLAGS = [
+    "-C"
+    "linker=${targetCc}"
+    "-L"
+    "native=${pkgsWin32.windows.pthreads}/lib"
+  ];
+
+  CC = "${targetCc}";
+  LD = "${targetCc}";
+}