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-lib.el

@@ -224,9 +224,7 @@ BODY is evaluated once TARGETS are loaded. TARGETS can either be:
               #'progn
             #'with-no-warnings)
           (if (symbolp targets)
-              `(progn
-                 (doom-module-register-config ',targets ,(FILE!))
-                 (with-eval-after-load ',targets ,@body))
+              `(with-eval-after-load ',targets ,@body)
             (pcase (car-safe targets)
               ((or :or :any)
                (macroexp-progn