plover.stable: move deprecation behind allowAliases (NixOS#395787)

Author: jopejoe1

pkgs/applications/misc/plover/default.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  config,
   fetchFromGitHub,
   python3Packages,
   wmctrl,
@@ -8,8 +9,6 @@
 }:
 
 {
-  stable = throw "plover.stable was removed because it used Python 2. Use plover.dev instead."; # added 2022-06-05
-
   dev =
     with python3Packages;
     mkDerivationWith buildPythonPackage rec {
@@ -58,3 +57,6 @@
       '';
     };
 }
+// lib.optionalAttrs config.allowAliases {
+  stable = throw "plover.stable was removed because it used Python 2. Use plover.dev instead."; # added 2022-06-05
+}