refactor: split flake.nix into muiltple files

nix/libdeltachat.nix

@@ -0,0 +1,26 @@
+{ pkgs, fenixToolchain, rustSrc, cargoLock, fenixPkgs, version }:
+let
+  rustPlatform = (pkgs.makeRustPlatform {
+    cargo = fenixToolchain;
+    rustc = fenixToolchain;
+  });
+in
+pkgs.stdenv.mkDerivation {
+  pname = "libdeltachat";
+  inherit version;
+  src = rustSrc;
+  cargoDeps = pkgs.rustPlatform.importCargoLock cargoLock;
+
+  nativeBuildInputs = [
+    pkgs.perl # Needed to build vendored OpenSSL.
+    pkgs.cmake
+    rustPlatform.cargoSetupHook
+    fenixPkgs.stable.rustc
+    fenixPkgs.stable.cargo
+  ];
+
+  postInstall = ''
+    substituteInPlace $out/include/deltachat.h \
+      --replace __FILE__ '"${placeholder "out"}/include/deltachat.h"'
+  '';
+}