refactor: split flake.nix into multiple files

nix/wheel.nix

@@ -0,0 +1,28 @@
+{ pkgs, nix-filter, version, root }:
+{ rpc-server, arch, binaryName ? "deltachat-rpc-server" }:
+pkgs.stdenv.mkDerivation {
+  pname = "deltachat-rpc-server-${arch}-wheel";
+  inherit version;
+  src = nix-filter.lib {
+    inherit root;
+    include = [
+      "scripts/wheel-rpc-server.py"
+      "deltachat-rpc-server/README.md"
+      "LICENSE"
+      "Cargo.toml"
+    ];
+  };
+  nativeBuildInputs = [
+    pkgs.python3
+    pkgs.python3Packages.wheel
+  ];
+  buildInputs = [
+    rpc-server
+  ];
+  buildPhase = ''
+    mkdir tmp
+    cp ${rpc-server}/bin/${binaryName} tmp/${binaryName}
+    python3 scripts/wheel-rpc-server.py ${arch} tmp/${binaryName}
+  '';
+  installPhase = ''mkdir -p $out; cp -av deltachat_rpc_server-*.whl $out'';
+}