emacs: do not use seq package from elpa for emacs >= 27

The `seq` package that comes with emacs 27 is newer than that provided
by ELPA. Various packages list `seq` as an input, causing it to be
imported from ELPA in addition to the (newer) version bundled with
emacs. Relying on the version packaged with emacs resolves the
conflict.

Fixes #73346

pkgs/applications/editors/emacs-modes/elpa-packages.nix

@@ -40,6 +40,9 @@ self: let
       cl-lib = null; # builtin
       tle = null; # builtin
       advice = null; # builtin
+      seq = if lib.versionAtLeast self.emacs.version "27"
+            then null
+            else super.seq;
     };
 
     elpaPackages = super // overrides;