emacs: correct the order of profiles and their sub dirs in load-path

This patch does two things:
1. making user profiles preferred over system profiles
2. putting sub dirs of one profile to the right place
  - before this patch, they are appended to the end of load-path
  - after this patch, they are inserted right after the profile

Example value of load-path before this patch:

  /run/current-system/sw/share/emacs/site-lisp/
  /etc/profiles/per-user/user/share/emacs/site-lisp/
  /nix/store/hash1-emacs-packages-deps/share/emacs/site-lisp
  /nix/store/hash1-emacs-packages-deps/share/emacs/site-lisp/elpa
  /nix/store/hash1-emacs-packages-deps/share/emacs/site-lisp/elpa/wgrep-20230203.1214
  /nix/store/hash2-emacs-29.1-rc1/share/emacs/29.1/site-lisp
  /nix/store/hash2-emacs-29.1-rc1/share/emacs/site-lisp
  /nix/store/hash2-emacs-29.1-rc1/share/emacs/29.1/lisp
  /nix/store/hash2-emacs-29.1-rc1/share/emacs/29.1/lisp/vc
  ...
  /etc/profiles/per-user/user/share/emacs/site-lisp/elpa
  /etc/profiles/per-user/user/share/emacs/site-lisp/elpa/jinx-20230730.1200
  /run/current-system/sw/share/emacs/site-lisp/elpa
  /run/current-system/sw/share/emacs/site-lisp/elpa/repology-1.2.3

after this patch:

  /etc/profiles/per-user/user/share/emacs/site-lisp
  /etc/profiles/per-user/user/share/emacs/site-lisp/elpa
  /etc/profiles/per-user/user/share/emacs/site-lisp/elpa/jinx-20230730.1200
  /run/current-system/sw/share/emacs/site-lisp
  /run/current-system/sw/share/emacs/site-lisp/elpa
  /run/current-system/sw/share/emacs/site-lisp/elpa/repology-1.2.3
  /nix/store/hash1-emacs-packages-deps/share/emacs/site-lisp
  /nix/store/hash1-emacs-packages-deps/share/emacs/site-lisp/elpa
  /nix/store/hash1-emacs-packages-deps/share/emacs/site-lisp/elpa/wgrep-20230203.1214
  /nix/store/hash2-emacs-29.1-rc1/share/emacs/29.1/site-lisp
  /nix/store/hash2-emacs-29.1-rc1/share/emacs/site-lisp
  /nix/store/hash2-emacs-29.1-rc1/share/emacs/29.1/lisp
  /nix/store/hash2-emacs-29.1-rc1/share/emacs/29.1/lisp/vc
  ...

1 files changed, 5 insertions, 2 deletions

diff --git a/pkgs/applications/editors/emacs/site-start.el b/pkgs/applications/editors/emacs/site-start.el
index acc6833b98c9d..668b76cfd3e80 100644
--- a/pkgs/applications/editors/emacs/site-start.el
+++ b/pkgs/applications/editors/emacs/site-start.el
@@ -8,8 +8,11 @@ least specific (the system profile)"
 ;;; Extend `load-path' to search for elisp files in subdirectories of all folders in `NIX_PROFILES'.
 ;;; Non-Nix distros have similar logic in /usr/share/emacs/site-lisp/subdirs.el.
 ;;; See https://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Search.html
-(dolist (profile (nix--profile-paths))
-  (let ((default-directory (expand-file-name "share/emacs/site-lisp/" profile)))
+(dolist (profile (reverse (nix--profile-paths)))
+  ;; `directory-file-name' is important to add sub dirs to the right place of `load-path'
+  ;; see the source code of `normal-top-level-add-to-load-path'
+  (let ((default-directory (directory-file-name
+                            (expand-file-name "share/emacs/site-lisp/" profile))))
     (when (file-exists-p default-directory)
       (setq load-path (cons default-directory load-path))
       (normal-top-level-add-subdirs-to-load-path))))