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

early-init.el

@@ -133,8 +133,10 @@
           ;;   fit guess. It's better than Emacs' 80kb default.
           (setq gc-cons-threshold (* 16 1024 1024))
           nil))
-       ;; ...Otherwise, we're loading a Doom config, so continue as normal.
-      (doom-require (if noninteractive 'doom-cli 'doom-start))
+      ;; In non-interactive sessions, leave to the consumer to call
+      ;; `doom-initialize' at the best time, otherwise we need to initialize
+      ;; ASAP for the Emacs session ahead.
+      (doom-initialize (not noninteractive))
     ;; If we're here, the user wants to load another config/profile (that may or
     ;; may not be a Doom config).
     (load user-init-file 'noerror (not init-file-debug) nil 'must-suffix)))