refactor!(emacs-lisp): flycheck config in non-packages

BREAKING CHANGE: This performs the following backwards-incompatible
changes:

- Replaces `+emacs-lisp-reduce-flycheck-errors-in-emacs-config-h` with a
  `+emacs-lisp-non-package-mode` minor-mode.
- Removed the `+emacs-lisp-disable-flycheck-in-dirs` variable, as this
  mechanism no longer checks a directory list to detect a "non-package".

If you've referenced either of these symbols, you'll need to
update/remove them from your config. No extra config is needed
otherwise.

Why: Doom has always tried to reduce the verbosity of Flycheck when
viewing elisp config files or scripts (i.e. non-packages). These are so
stateful that the byte-compiler, package-lint, and checkdoc inundate
users with false positives that are more overwhelming than helpful.

The heuristic for this has always been a simple "is this file in
$DOOMDIR or $EMACSDIR", but this wasn't robust enough, especially in
cases where symlinking was involved, so I've employed a new, more
general heuristic for detecting non-package files:

- The file isn't a theme in `custom-theme-load-path`,
- The file doesn't have a (provide ...) or (provide-theme ...)
  statement whose first argument matches the file name,
- The file lives in a project with a .doommodule file (doom modules
  never have convention package files in them),
- Or the file is a dotfile (like .dir-locals.el or .doomrc).

I've also tweaked byte-compile-warnings to yield a little more output,
but not by much. Whether this is too permissive or not will require
further testing to determine.

What's more, I've updated this to reflect recent changes to Doom's
startup process (in c05e615).

Ref: c05e61536e

modules/lang/emacs-lisp/autoload.el

@@ -244,33 +244,70 @@ https://emacs.stackexchange.com/questions/10230/how-to-indent-keywords-aligned"
           ("Variables" "^\\s-*(\\(def\\(?:c\\(?:onst\\(?:ant\\)?\\|ustom\\)\\|ine-symbol-macro\\|parameter\\|var\\(?:-local\\)?\\)\\)\\s-+\\(\\(?:\\sw\\|\\s_\\|\\\\.\\)+\\)" 2)
           ("Types" "^\\s-*(\\(cl-def\\(?:struct\\|type\\)\\|def\\(?:class\\|face\\|group\\|ine-\\(?:condition\\|error\\|widget\\)\\|package\\|struct\\|t\\(?:\\(?:hem\\|yp\\)e\\)\\)\\)\\s-+'?\\(\\(?:\\sw\\|\\s_\\|\\\\.\\)+\\)" 2))))
 
+(defun +emacs-lisp--in-package-buffer-p ()
+  (let* ((file-path (buffer-file-name (buffer-base-buffer)))
+         (file-base (if file-path (file-name-base file-path))))
+    (and (derived-mode-p 'emacs-lisp-mode)
+         (or (null file-base)
+             (locate-file file-base (custom-theme--load-path) '(".elc" ".el"))
+             (save-excursion
+               (save-restriction
+                 (widen)
+                 (goto-char (point-max))
+                 (when (re-search-backward "^ *\\((provide\\)\\(?:-theme\\)? +'"
+                                           (max (point-min) (- (point-max) 512))
+                                           t)
+                   (goto-char (match-beginning 1))
+                   (ignore-errors
+                     (and (stringp file-base)
+                          (equal (symbol-name (doom-unquote (nth 1 (read (current-buffer)))))
+                                 file-base)))))))
+         (not (locate-dominating-file default-directory ".doommodule")))))
+
 ;;;###autoload
-(defun +emacs-lisp-reduce-flycheck-errors-in-emacs-config-h ()
-  "Remove `emacs-lisp-checkdoc' checker and reduce `emacs-lisp' checker
-verbosity when editing a file in `doom-user-dir' or `doom-emacs-dir'."
-  (when (and (bound-and-true-p flycheck-mode)
-             (eq major-mode 'emacs-lisp-mode)
-             (or (not default-directory)
-                 (null (buffer-file-name (buffer-base-buffer)))
-                 (cl-find-if (doom-partial #'file-in-directory-p default-directory)
-                             +emacs-lisp-disable-flycheck-in-dirs)))
-    (add-to-list 'flycheck-disabled-checkers 'emacs-lisp-checkdoc)
-    (set (make-local-variable 'flycheck-emacs-lisp-check-form)
-         (concat "(progn "
-                 (prin1-to-string
-                  `(ignore-errors
-                     (setq doom-modules ',doom-modules
-                           doom-disabled-packages ',doom-disabled-packages)
-                     (require 'doom)
-                     (ignore-errors (load ,user-init-file t t))
-                     (setq byte-compile-warnings
-                           '(obsolete cl-functions
-                             interactive-only make-local mapcar
-                             suspicious constants))
-                     (defmacro map! (&rest _))))
-                 " "
-                 (default-value 'flycheck-emacs-lisp-check-form)
-                 ")"))))
+(define-minor-mode +emacs-lisp-non-package-mode
+  "Reduce flycheck verbosity where it is appropriate.
+
+Essentially, this means in any elisp file that either:
+- Is not a theme in `custom-theme-load-path',
+- Lacks a `provide' statement,
+- Lives in a project with a .doommodule file,
+- Is a dotfile (like .dir-locals.el or .doomrc).
+
+This generally applies to your private config (`doom-user-dir') or Doom's source
+\(`doom-emacs-dir')."
+  :since "3.0.0"
+  (unless (and (bound-and-true-p flycheck-mode)
+               (not (+emacs-lisp--in-package-buffer-p)))
+    (setq +emacs-lisp-non-package-mode nil))
+  (when (derived-mode-p 'emacs-lisp-mode)
+    (add-hook 'after-save-hook #'+emacs-lisp-non-package-mode nil t))
+  (if (not +emacs-lisp-non-package-mode)
+      (when (get 'flycheck-disabled-checkers 'initial-value)
+        (setq-local flycheck-disabled-checkers (get 'flycheck-disabled-checkers 'initial-value))
+        (kill-local-variable 'flycheck-emacs-lisp-check-form))
+    (with-memoization (get 'flycheck-disabled-checkers 'initial-value)
+      flycheck-disabled-checkers)
+    (setq-local flycheck-emacs-lisp-check-form
+                (prin1-to-string
+                 `(progn
+                    (setq doom-modules ',doom-modules
+                          doom-disabled-packages ',doom-disabled-packages
+                          byte-compile-warnings ',+emacs-lisp-linter-warnings)
+                    (condition-case e
+                        (progn
+                          (require 'doom)
+                          (require 'doom-cli)
+                          (require 'doom-start)
+                          (defmacro map! (&rest _)))
+                      (error
+                       (princ
+                        (format "%s:%d:%d:Error:Failed to load Doom: %s\n"
+                                ,(file-name-nondirectory (buffer-file-name (buffer-base-buffer)))
+                                0 0 (error-message-string e)))))
+                    ,(read (default-toplevel-value 'flycheck-emacs-lisp-check-form))))
+                flycheck-disabled-checkers (cons 'emacs-lisp-checkdoc
+                                                 flycheck-disabled-checkers))))
 
 
 ;;