refactor: split flake.nix into multiple files

nix/win64-package.nix

@@ -0,0 +1,47 @@
+{ pkgs, naersk, fenixPkgs, system, version }:
+packageName:
+let
+  pkgsWin64 = pkgs.pkgsCross.mingwW64;
+  rustTarget = pkgsWin64.stdenv.hostPlatform.rust.rustcTarget;
+  toolchainWin = fenixPkgs.combine [
+    fenixPkgs.stable.rustc
+    fenixPkgs.stable.cargo
+    fenixPkgs.targets.${rustTarget}.stable.rust-std
+  ];
+  naerskWin = pkgs.callPackage naersk {
+    cargo = toolchainWin;
+    rustc = toolchainWin;
+  };
+  targetCc = "${pkgsWin64.stdenv.cc}/bin/${pkgsWin64.stdenv.cc.targetPrefix}cc";
+in
+naerskWin.buildPackage {
+  pname = packageName;
+  cargoBuildOptions = x: x ++ [ "--package" packageName ];
+  inherit version;
+  strictDeps = true;
+  src = pkgs.lib.cleanSource ../.;
+  nativeBuildInputs = [
+    pkgs.perl # Needed to build vendored OpenSSL.
+  ];
+  depsBuildBuild = [
+    pkgsWin64.stdenv.cc
+  ];
+  buildInputs = [
+    pkgsWin64.windows.pthreads
+  ];
+  auditable = false; # Avoid cargo-auditable failures.
+  doCheck = false; # Disable test as it requires network access.
+
+  CARGO_BUILD_TARGET = rustTarget;
+  TARGET_CC = "${targetCc}";
+  CFLAGS_x86_64_pc_windows_gnu = "-I${pkgsWin64.windows.pthreads}/include";
+  CARGO_BUILD_RUSTFLAGS = [
+    "-C"
+    "linker=${targetCc}"
+    "-L"
+    "native=${pkgsWin64.windows.pthreads}/lib"
+  ];
+
+  CC = "${targetCc}";
+  LD = "${targetCc}";
+}