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/lib/help.el

@@ -533,7 +533,6 @@ If prefix arg is present, refresh the cache."
                           packages nil t nil nil
                           (when guess (symbol-name guess))))))))
   ;; TODO Refactor me.
-  (require 'doom-packages)
   (doom-initialize-packages)
   (help-setup-xref (list #'doom/help-packages package)
                    (called-interactively-p 'interactive))