refactor!: restructure Doom core

BREAKING CHANGE: This restructures Doom's core in an effort to slim it
down and partially mirror architectural changes coming in v3. This is
part 2 of 3 commits (part 1 being 1590434), done to facilitate a change
in part 3 that will introduce a new `doom!` syntax for pulling
third-party module libraries from remote sources (similar to `package!`
statements). I am backporting this from V3 so I can move our modules out
into separate repos sooner than later, so development on modules can
continue separately without interfering with v3's roll out.

Though this is labeled a breaking change, it shouldn't affect most users
except those few tinkering directly with Doom's internals.

Ref: 15904349cf

lisp/doom-elpaca.el

@@ -0,0 +1,12 @@
+;;; lisp/doom-elpaca.el -*- lexical-binding: t; -*-
+;;; Commentary:
+;;
+;; In the near future, Doom will replace Straight with Elpaca. Its configuration
+;; will live here.
+;;
+;;; Code:
+
+;; TODO
+
+(provide 'doom-elpaca)
+;;; doom-elpaca.el ends here.