Refactor doom-initialize-packages & package API

- Packages are initialized once, when package.el is first loaded, and
  must be updated manually via doom/reload-packages.
- Package->module association is now stored in the package's PLIST under
  :modules. This is an internal property and cannot be explicitly set
  through `package!'
- Add doom-package-list function
- Rename doom-get-packages to doom-find-packages
- Updated doom-find-packages' docstring
- Added the :core filter to doom-find-packages
- Simplified doom-initialize-packages
- doom/reload calls doom/reload-packages if necessary.
- Fix redundant properties in doom-packages
- Remove tracking of after!, def-package! and def-package-hook! blocks.
  Replaced with doom-package-list being able to see all packages, even
  in disabled modules.
- Add :built-in property to package! for dummy packages. This is
  important so that doom/describe-package can see built-in packages.

core/core-modules.el

@@ -188,15 +188,6 @@ non-nil, return paths of possible modules, activated or otherwise."
                      collect (plist-get plist :path)))
           (list doom-private-dir)))
 
-(defun doom-module-register-config (package file &optional append)
-  "TODO"
-  (let ((files (get package 'doom-files)))
-    (unless (member file files)
-      (if append
-          (setq files (append files (list file)))
-        (push file files))
-      (put package 'doom-files files))))
-
 (defun doom-modules (&optional refresh-p)
   "Minimally initialize `doom-modules' (a hash table) and return it."
   (or (unless refresh-p doom-modules)
@@ -372,9 +363,7 @@ package is disabled."
               ;; package errors, so we preform this check at compile time:
               (and (bound-and-true-p byte-compile-current-file)
                    (not (locate-library (symbol-name name)))))
-    `(progn
-       (doom-module-register-config ',name ,(FILE!) t)
-       (use-package ,name ,@plist))))
+    `(use-package ,name ,@plist)))
 
 (defmacro def-package-hook! (package when &rest body)
   "Reconfigures a package's `def-package!' block.
@@ -396,7 +385,6 @@ to have them return non-nil (or exploit that to overwrite Doom's config)."
     (error "'%s' isn't a valid hook for def-package-hook!" when))
   `(progn
      (setq use-package-inject-hooks t)
-     (doom-module-register-config ',package ,(FILE!))
      (add-hook!
        ',(intern (format "use-package--%s--%s-hook"
                          package