nix: make usage in non-flake setups easier (#296)

* nix: extract overlay into own file
* readme: document non-flake nixos usage

README.md

@@ -2344,7 +2344,7 @@ some recommended dependencies are enabled by default; [override the package](htt
 
 ## nixos module
 
-for this setup, you will need a [flake-enabled](https://nixos.wiki/wiki/Flakes) installation of NixOS.
+for [flake-enabled](https://nixos.wiki/wiki/Flakes) installations of NixOS:
 
 ```nix
 {
@@ -2371,6 +2371,33 @@ for this setup, you will need a [flake-enabled](https://nixos.wiki/wiki/Flakes)
 }
 ```
 
+if you don't use a flake in your configuration, you can use other dependency management tools like [npins](https://github.com/andir/npins), [niv](https://github.com/nmattia/niv), or even plain [`fetchTarball`](https://nix.dev/manual/nix/stable/language/builtins#builtins-fetchTarball), like so:
+
+```nix
+{ pkgs, ... }:
+
+let
+  # npins example, adjust for your setup. copyparty should be a path to the downloaded repo
+  # for niv, just replace the npins folder import with the sources.nix file
+  copyparty = (import ./npins).copyparty;
+
+  # or with fetchTarball:
+  copyparty = fetchTarball "https://github.com/9001/copyparty/archive/hovudstraum.tar.gz";
+in
+
+{
+  # load the copyparty NixOS module
+  imports = [ "${copyparty}/contrib/nixos/modules/copyparty.nix" ];
+
+  # add the copyparty overlay to expose the package to the module
+  nixpkgs.overlays = [ "${copyparty}/contrib/package/nix/overlay.nix" ];
+  # (optional) install the package globally
+  environment.systemPackages = [ pkgs.copyparty ];
+  # configure the copyparty module
+  services.copyparty.enable = true;
+}
+```
+
 copyparty on NixOS is configured via `services.copyparty` options, for example:
 ```nix
 services.copyparty = {

contrib/package/nix/overlay.nix

@@ -0,0 +1,11 @@
+final: prev: {
+  copyparty = final.python3.pkgs.callPackage ./copyparty {
+    ffmpeg = final.ffmpeg-full;
+  };
+
+  python3 = prev.python3.override {
+    packageOverrides = pyFinal: pyPrev: {
+      partftpy = pyFinal.callPackage ./partftpy { };
+    };
+  };
+}

flake.nix

@@ -12,16 +12,7 @@
     }:
     {
       nixosModules.default = ./contrib/nixos/modules/copyparty.nix;
-      overlays.default = final: prev: {
+      overlays.default = import ./contrib/package/nix/overlay.nix;
-        copyparty = final.python3.pkgs.callPackage ./contrib/package/nix/copyparty {
-          ffmpeg = final.ffmpeg-full;
-        };
-        python3 = prev.python3.override {
-          packageOverrides = pyFinal: pyPrev: {
-            partftpy = pyFinal.callPackage ./contrib/package/nix/partftpy { };
-          };
-        };
-      };
     }
     // flake-utils.lib.eachDefaultSystem (
       system: