refactor!: restructure Doom core

BREAKING CHANGE: This restructures the project in preparation for Doom
to be split into two repos. Users that have reconfigured Doom's CLI
stand a good chance of seeing breakage, especially if they've referred
to any core-* feature, e.g.

  (after! core-cli-ci ...)

To fix it, simply s/core-/doom-/, i.e.

  (after! doom-cli-ci ...)

What this commit specifically changes is:
- Renames all core features from core-* to doom-*
- Moves core/core-* -> lisp/doom-*
- Moves core/autoloads/* -> lisp/lib/*
- Moves core/templates -> templates/

Ref: #4273

modules/lang/emacs-lisp/autoload.el

@@ -261,8 +261,8 @@ verbosity when editing a file in `doom-private-dir' or `doom-emacs-dir'."
                   `(progn
                      (setq doom-modules ',doom-modules
                            doom-disabled-packages ',doom-disabled-packages)
-                     (require 'core)
-                     (require 'core-cli-lib)
+                     (require 'doom)
+                     (require 'doom-cli-lib)
                      (ignore-errors (load ,user-init-file t t))
                      (setq byte-compile-warnings
                            '(obsolete cl-functions