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-projects.el

@@ -0,0 +1,40 @@
+;; -*- no-byte-compile: t; -*-
+;;; core/test/test-core-projects.el
+
+(describe "core/projects"
+  :var (projectile-enable-caching)
+
+  (require 'core-projects)
+  (require 'projectile)
+
+  (before-each
+    (setq projectile-enable-caching nil)
+    (projectile-mode +1))
+  (after-each
+    (projectile-mode -1))
+
+  (describe "project-p"
+    (it "Should detect when in a valid project"
+      (expect (doom-project-p doom-emacs-dir)))
+    (it "Should detect when not in a valid project"
+      (expect (doom-project-p (expand-file-name "~")) :to-be nil)))
+
+  (describe "project-root"
+    (it "should resolve to the project's root"
+      (expect (doom-project-root doom-core-dir) :to-equal-file doom-emacs-dir))
+    (it "should return nil if not in a project"
+      (expect (doom-project-root (expand-file-name "~")) :to-be nil)))
+
+  (describe "project-expand"
+    (it "expands to a path relative to the project root"
+      (expect (doom-project-expand "init.el" doom-core-dir) :to-equal-file
+              (expand-file-name "init.el" (doom-project-root doom-core-dir)))))
+
+  (describe "project-file-exists-p!"
+    (let ((default-directory doom-core-dir))
+      ;; Resolve from project root
+      (expect (project-file-exists-p! "init.el"))
+      ;; Chained file checks
+      (expect (project-file-exists-p! (and "init.el" "LICENSE")))
+      (expect (project-file-exists-p! (or "init.el" "does-not-exist")))
+      (expect (project-file-exists-p! (and "init.el" (or "LICENSE" "does-not-exist")))))))