lang/org: refactor contrib modules

- Fix #1546
- Ensure contrib modules aren't included in byte-compilation/autoload
  generation if their requisite flags aren't enabled

modules/lang/org/contrib/present.el

@@ -1,4 +1,5 @@
 ;;; lang/org/contrib/present.el -*- lexical-binding: t; -*-
+;;;###if (featurep! +present)
 
 (defvar +org-present-text-scale 7
   "The `text-scale-amount' for `org-tree-slide-mode'.")