Enable read-only-mode in package files

These files shouldn't be modified directly.

modules/lang/emacs-lisp/autoload.el

@@ -148,3 +148,12 @@ verbosity when editing a file in `doom-private-dir' or `doom-emacs-dir'."
                  " "
                  (default-value 'flycheck-emacs-lisp-check-form)
                  ")"))))
+
+;;;###autoload
+(defun +emacs-lisp|read-only-packages ()
+  "Enable `read-only-mode' if a package source file is opened.
+
+Packages are in `doom-packages-dir'. They shouldn't be modified directly."
+  (when-let* ((filename (buffer-file-name (buffer-base-buffer))))
+    (when (file-in-directory-p filename doom-packages-dir)
+      (read-only-mode +1))))

modules/lang/emacs-lisp/config.el

@@ -50,7 +50,8 @@ This marks a foldable marker for `outline-minor-mode' in elisp buffers.")
        rainbow-delimiters-mode
        highlight-quoted-mode
        ;; initialization
-       +emacs-lisp|extend-imenu))
+       +emacs-lisp|extend-imenu
+       +emacs-lisp|read-only-packages))
 
   ;; Flycheck's two emacs-lisp checkers produce a *lot* of false positives in
   ;; emacs configs, so we disable `emacs-lisp-checkdoc' and reduce the