refactor: rename :core module group to :doom

I'll soon introduce a new family of core modules which will house most
of what's currently in the doom-*.el libs, moving them out of core.

lisp/doom-profiles.el

@@ -412,7 +412,7 @@ Defaults to the profile at `doom-profile-default'."
               (let ((noextfile (file-name-sans-extension file)))
                 `(with-doom-module ',key
                    ,(pcase key
-                      ('(:core . nil)
+                      ('(:doom . nil)
                        `(doom-load
                          (file-name-concat
                           doom-core-dir ,(file-name-nondirectory noextfile))