refactor(lib): deprecate doom-dir

This function never really added much value over doom-path or
file-name-concat, so I'm phasing it out.

.doomrc

@@ -41,8 +41,8 @@
              #'doom-glob (if (string-prefix-p ":" scope)
                              (format "%s" (substring scope 1))
                            (format "*/%s" scope)))
-            (list (doom-dir (dir!) "../modules/")
-                  (doom-dir doom-user-dir "modules/"))))
+            (list (doom-path (dir!) "../modules/")
+                  (doom-path doom-user-dir "modules/"))))
 
 (defun ci-check-docs-scope (scope _)
   "Allow any filename in docs/* as a scope for docs commits."

lisp/cli/doctor.el

@@ -119,7 +119,7 @@ in."
   (let* ((xdg-dir (concat (or (getenv "XDG_CONFIG_HOME")
                               "~/.config")
                           "/doom/"))
-         (doom-dir (or (getenv "DOOMDIR")
+         (doom-path (or (getenv "DOOMDIR")
                        "~/.doom.d/"))
          (dir (if (file-directory-p xdg-dir)
                   xdg-dir

lisp/lib/files.el

@@ -72,12 +72,7 @@ If the glob ends in a slash, only returns matching directories."
       (file-expand-wildcards path full?))))
 
 ;;;###autoload
-(defun doom-dir (&rest segments)
-  "Constructs a path from SEGMENTS.
-See `doom-path'.
-Ignores `nil' elements in SEGMENTS."
-  (when-let (path (doom-path segments))
-    (directory-file-name path)))
+(define-obsolete-function-alias 'doom-dir 'doom-path "3.0.0")
 
 ;;;###autoload
 (cl-defun doom-files-in