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

test/test-core-modules.el

@@ -0,0 +1,23 @@
+;; -*- no-byte-compile: t; -*-
+;;; core/test/test-core-modules.el
+
+(xdescribe "core-modules"
+  (require 'core-modules)
+
+  (describe "doom!")
+  (describe "doom-modules")
+
+  (describe "doom-module-p")
+  (describe "doom-module-get")
+  (describe "doom-module-put")
+  (describe "doom-module-set")
+  (describe "doom-module-path")
+  (describe "doom-module-locate-path")
+  (describe "doom-module-from-path")
+  (describe "doom-module-load-path")
+
+  (describe "require!")
+  (describe "featurep!")
+  (describe "after!")
+  (describe "use-package!")
+  (describe "use-package-hook!"))