emacs.pkgs.evil-ghostel: use same source as ghostel

evil-ghostel and ghostel are maintained in the same repository as
ghostel, and they are tightly coupled (evil-ghostel advices functions
internal to ghostel): it can break on version skew. Since ghostel is a
manual package, they do not automatically remain in sync in nixpkgs even
if they are in sync upstream.

Fix this by using the src from ghostel to build evil-ghostel.

The recipe used to build evil-ghostel is just
`:files ("extensions/evil-ghostel/evil-ghostel.el")`: it will probably
be sufficiently stable for this approach to work.

Fixes #544713.

pkgs/applications/editors/emacs/elisp-packages/melpa-packages.nix

@@ -1174,6 +1174,9 @@ let
           # https://github.com/PythonNut/evil-easymotion/issues/74
           evil-easymotion = addPackageRequires super.evil-easymotion [ self.evil ];
 
+          # tightly coupled to ghostel (from manual-packages), use the same source
+          evil-ghostel = externalSrc super.evil-ghostel self.ghostel;
+
           evil-mu4e = addPackageRequires super.evil-mu4e [ self.mu4e ];
 
           # https://github.com/VanLaser/evil-nl-break-undo/issues/2