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/autoload/config.el

@@ -32,5 +32,7 @@ init.el and config.el. Then runs `doom-reload-hook'."
     (doom-initialize))
   (with-demoted-errors "PRIVATE CONFIG ERROR: %s"
     (doom-initialize-modules 'force))
+  (when (bound-and-true-p doom-packages)
+    (doom/reload-packages))
   (run-hook-wrapped 'doom-reload-hook #'doom-try-run-hook)
   (message "Finished!"))