refactor: doom-module: conform variables to conventions

- Rename doom--empty-module-context to doom-module--empty-context.
- Nest doom-module-context symbol plist in its 'keys' property. (By
  convention, Doom uses keywords as keys in most places. Let's be
  consistent)
- Updates all uses of doom-module--context-field and
  doom-module-context-get.

modules/lang/org/config.el

@@ -1352,7 +1352,7 @@ between the two."
       ))
 
   ;;; Custom org modules
-  (dolist (flag (doom-module-context-get 'flags))
+  (dolist (flag (doom-module-context-get :flags))
     (load! (concat "contrib/" (substring (symbol-name flag) 1)) nil t))
 
   ;; Add our general hooks after the submodules, so that any hooks the