nix: refactor

- Remove unnecessary fenix input
- Remove unnecessary arguments to buildRustPackage
- Remove obsolete/unnecessary hacks
- Minimize code duplication and maximize re-use by using flake.parts
- Split out stardust-xr-server derivation into its own nix file in nix/stardust-xr-server
- Automatically get name of package from Cargo.toml
- Advertise support for riscv64-linux in flake outputs

nix/stardust-xr-server.nix

@@ -0,0 +1,40 @@
+{ rustPlatform
+, src
+, name
+, openxr-loader
+, libGL
+, mesa
+, xorg
+, fontconfig
+, libxkbcommon
+, libclang
+, cmake
+, cpm-cmake
+, pkg-config
+, llvmPackages
+}:
+
+rustPlatform.buildRustPackage rec {
+  inherit src name;
+  cargoLock = {
+    lockFile = (src + "/Cargo.lock");
+    allowBuiltinFetchGit = true;
+  };
+  CPM_SOURCE_CACHE = "./build";
+  postPatch = ''
+    sk=$(echo $cargoDepsCopy/stereokit-sys-*/StereoKit)
+    mkdir -p $sk/build/cpm
+
+    # This is not ideal, the original approach was to fetch the exact cmake
+    # file version that was wanted from GitHub directly, but at least this way it comes from Nixpkgs.. so meh
+    cp ${cpm-cmake}/share/cpm/CPM.cmake $sk/build/cpm/CPM_0.32.2.cmake
+  '';
+  nativeBuildInputs = [
+    cmake pkg-config llvmPackages.libcxxClang
+  ];
+  buildInputs = [
+    openxr-loader libGL mesa xorg.libX11 fontconfig libxkbcommon
+  ];
+  LIBCLANG_PATH = "${libclang.lib}/lib";
+}
+