fix: add :depth field to modules

This introduces a depth field for modules so that they may dictate their
load order explicitly, it also treats depths <= -100 or >= 100 as
special depths, which will be loaded early, before their respective
doom-{before,after}-module-{init,config}-hook. This permits psuedo
modules like :core and :user modules to be treated as normal modules
without too many special cases.

This also fixes a module load order issue on Emacs 29 (#6813), caused by
emacs-mirror/emacs@4311bd0bd7, which changed the return value order of
hash-table-{keys,values} causing modules to be loaded in reverse order;
resulting in the loss of evil keybinds, among other things.

Other notable changes:
- Changes the data structure for module data caches from a list to a
  vector. Uses less memory and permits faster lookups. Also adds two
  depth fields to the front of it.
- Changes the signature of doom-module-list and doom-package-list.
- Renames doom--read-packages -> doom-packages--read for consistency
  with naming convention.
- Add doom-module-depth function.
- Adds a temporary doom-core-dir/init.el file, which is responsible for
  loading doom-*.el.

Fix: #6813
Ref: emacs-mirror/emacs@4311bd0bd7

lisp/init.el

@@ -0,0 +1,16 @@
+;;; lisp/init.el -*- lexical-binding: t; -*-
+;;; Commentary:
+;;
+;; :core is now treated like a normal module, and this is its (temporary) init
+;; file, which will be removed once we've resolved our `use-package' dependency
+;; (which will soon be moved to its own module), then these will be returned to
+;; the profile init file.
+;;
+;;; Code:
+
+(doom-require 'doom-keybinds)
+(doom-require 'doom-ui)
+(doom-require 'doom-projects)
+(doom-require 'doom-editor)
+
+;;; init.el ends here