gnu: emacs: Wrap EMACSLOADPATH.

With this, the search path specification of EMACSLOADPATH does no longer
depend on the version of Emacs, which should make upgrading major versions
less painful.  See also:
- <https://bugs.gnu.org/43627>
- <https://bugs.gnu.org/47458>

* gnu/packages/emacs.scm (emacs)[#:phases]: Add ‘wrap-load-path’.
[native-search-path]<EMACSLOADPATH>: Do not search for builtin libraries.
(emacs-next)[native-search-path]: Inherit from emacs.

gnu/packages/emacs.scm

@@ -201,6 +201,24 @@
                 (car (find-files "bin" "^emacs-([0-9]+\\.)+[0-9]+$"))
                 "bin/emacs")
                #t)))
+         (add-after 'strip-double-wrap 'wrap-load-path
+           (lambda* (#:key outputs #:allow-other-keys)
+             (let* ((out (assoc-ref outputs "out"))
+                    (lisp-dirs (find-files (string-append out "/share/emacs")
+                                           "^lisp$"
+                                           #:directories? #t)))
+               (for-each
+                (lambda (prog)
+                  (wrap-program prog
+                    `("EMACSLOADPATH" suffix ,lisp-dirs)))
+                (find-files (string-append out "/bin")
+                            ;; Matches versioned and unversioned emacs binaries.
+                            ;; We don't patch emacsclient, because it takes its
+                            ;; environment variables from emacs.
+                            ;; Likewise, we don't need to patch helper binaries
+                            ;; like etags, ctags or ebrowse.
+                            "^emacs(-[0-9]+(\\.[0-9]+)*)?$"))
+               #t)))
          (add-before 'reset-gzip-timestamps 'make-compressed-files-writable
            ;; The 'reset-gzip-timestamps phase will throw a permission error
            ;; if gzip files aren't writable then.  This phase is needed when
@@ -255,9 +273,7 @@
     (native-search-paths
      (list (search-path-specification
             (variable "EMACSLOADPATH")
-            ;; The versioned entry is for the Emacs' builtin libraries.
-            (files (list "share/emacs/site-lisp"
-                         (string-append "share/emacs/" version "/lisp"))))
+            (files '("share/emacs/site-lisp")))
            (search-path-specification
             (variable "INFOPATH")
             (files '("share/info")))))
@@ -294,18 +310,7 @@ languages.")
            "0igjm9kwiswn2dpiy2k9xikbdfc7njs07ry48fqz70anljj8y7y3"))))
       (native-inputs
        `(("autoconf" ,autoconf)
-         ,@(package-native-inputs emacs)))
-      (native-search-paths
-       (list (search-path-specification
-              (variable "EMACSLOADPATH")
-              ;; The versioned entry is for the Emacs' builtin libraries.
-              (files (list "share/emacs/site-lisp"
-                           (string-append "share/emacs/"
-                                          (version-major+minor+point version)
-                                          "/lisp"))))
-             (search-path-specification
-              (variable "INFOPATH")
-              (files '("share/info"))))))))
+         ,@(package-native-inputs emacs))))))
 
 (define-public emacs-next-pgtk
   (let ((commit "ae18c8ec4f0ef37c8c9cda473770ff47e41291e2")