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/debug.el

@@ -61,7 +61,7 @@ ready to be pasted in a bug report on github."
          "n/a")
      (or (ignore-errors
            (require 'use-package)
-           (cl-loop for (name . plist) in (doom-get-packages :private t)
+           (cl-loop for (name . plist) in (doom-find-packages :private t)
                     if (use-package-plist-delete (copy-sequence plist) :private)
                     collect (format "%s" (cons name it))
                     else