Create mopidy home-manager module

It is just an adapted version from the NixOS module. I'll eventually
figure out how to be 'properly configured' with the Nix language through
the `lib.generator`.

modules/home-manager/services/mopidy.nix

@@ -0,0 +1,88 @@
+# This service is adapted from the NixOS module.
+{ config, options, lib, pkgs, ... }:
+
+let
+  cfg = config.services.mopidy;
+
+  mopidyConf = pkgs.writeText "mopidy.conf" cfg.configuration;
+  mopidyEnv = pkgs.buildEnv {
+    name = "mopidy-with-extensions-${pkgs.mopidy.version}";
+    # TODO: Improve this that doesn't use `lib.misc`.
+    paths = lib.closePropagation cfg.extensionPackages;
+    pathsToLink = [ "/${pkgs.mopidyPackages.python.sitePackages}" ];
+    buildInputs = [ pkgs.makeWrapper ];
+    postBuild = ''
+      makeWrapper ${pkgs.mopidy}/bin/mopidy $out/bin/mopidy --prefix PYTHONPATH : $out/${pkgs.mopidyPackages.python.sitePackages}
+    '';
+  };
+in {
+  options.services.mopidy = {
+    enable = lib.mkEnableOption "Mopidy music player daemon";
+
+    dataDir = lib.mkOption {
+      default = "${config.xdg.dataHome}/mopidy";
+      type = with lib.types; either path str;
+      description = "The directory where Mopidy stores its state.";
+    };
+
+    extensionPackages = lib.mkOption {
+      default = [];
+      type = with lib.types; listOf package;
+      example = lib.literalExpression "with pkgs; [ mopidy-spotify mopidy-mpd mopidy-mpris ]";
+      description = ''
+        Mopidy extensions that should be loaded by the service.
+      '';
+    };
+
+    # TODO: Revise the configuration to a proper Nix setting.
+    configuration = lib.mkOption {
+      default = "";
+      type = lib.types.lines;
+      description = "The configuration Mopidy uses in the service.";
+    };
+
+    extraConfigFiles = lib.mkOption {
+      default = [];
+      type = with lib.types; listOf str;
+      description = ''
+        Extra config files to be read to the service.
+        Note that later files overrides earlier configuration.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      (lib.hm.assertions.assertPlatform "services.mopidy" pkgs lib.platforms.linux)
+    ];
+
+    systemd.user.services.mopidy = {
+      Unit = {
+        Description = "mopidy music player daemon";
+        After = [ "network.target" "sound.target" ];
+        Documentation = [ "https://mopidy.com/" ];
+      };
+
+      Service = {
+        ExecStart = "${mopidyEnv}/bin/mopidy --config ${lib.concatStringsSep ":" ([mopidyConf] ++ cfg.extraConfigFiles)}";
+      };
+
+      Install.WantedBy = [ "default.target" ];
+    };
+
+    systemd.user.services.mopidy-scan = {
+      Unit = {
+        After = [ "network.target" "sound.target" ];
+        Description = "mopidy local files scanner";
+        Documentation = [ "https://mopidy.com/" ];
+      };
+
+      Service = {
+        ExecStart = "${mopidyEnv}/bin/mopidy --config ${lib.concatStringsSep ":" ([mopidyConf] ++ cfg.extraConfigFiles)} local scan";
+        Type = "oneshot";
+      };
+
+      Install.WantedBy = [ "default.target" ];
+    };
+  };
+}