refactor(copyparty): simplify module helpers

configs/nixos/modules/services/copyparty.nix

@@ -7,42 +7,11 @@
 let
   cfg = config.modules.services.copyparty; # NOLINT(let_order)
 
-  volumeType = lib.types.submodule {
-    options = {
-      path = lib.mkOption {
-        type = lib.types.str;
-        description = "Filesystem path served by this volume.";
-      };
-
-      access = lib.mkOption {
-        type = lib.types.attrsOf (lib.types.listOf lib.types.str);
-        default = { };
-        example = {
-          A = [ "alexion" ];
-        };
-        description = "Copyparty access map from permission string to account names.";
-      };
-
-      readOnly = lib.mkOption {
-        type = lib.types.bool;
-        default = false;
-        description = "Serve this volume read-only.";
-      };
-
-      flags = lib.mkOption {
-        type = lib.types.listOf lib.types.str;
-        default = [ ];
-        example = [ "dthumb" ];
-        description = "Copyparty volume flags.";
-      };
-    };
-  };
-
   configScript = pkgs.writeShellScript "copyparty-config" ''
     set -eu
 
     umask 077
-    password="$(${pkgs.coreutils}/bin/tr -d '\n' < "$CREDENTIALS_DIRECTORY/password")"
+    password="$(${lib.getExe' pkgs.coreutils "tr"} -d '\n' < "$CREDENTIALS_DIRECTORY/password")"
 
     cat > /run/copyparty/copyparty.conf <<EOF
     [global]
@@ -140,7 +109,38 @@ in
     };
 
     volumes = lib.mkOption {
-      type = lib.types.attrsOf volumeType;
+      type = lib.types.attrsOf (
+        lib.types.submodule {
+          options = {
+            path = lib.mkOption {
+              type = lib.types.externalPath;
+              description = "Filesystem path served by this volume.";
+            };
+
+            access = lib.mkOption {
+              type = lib.types.attrsOf (lib.types.listOf lib.types.str);
+              default = { };
+              example = {
+                A = [ "alexion" ];
+              };
+              description = "Copyparty access map from permission string to account names.";
+            };
+
+            readOnly = lib.mkOption {
+              type = lib.types.bool;
+              default = false;
+              description = "Serve this volume read-only.";
+            };
+
+            flags = lib.mkOption {
+              type = lib.types.listOf lib.types.str;
+              default = [ ];
+              example = [ "dthumb" ];
+              description = "Copyparty volume flags.";
+            };
+          };
+        }
+      );
       default = { };
       description = "Copyparty volumes keyed by their URL path.";
     };