refactor!: complete profile gen and init systems

BREAKING CHANGE: This commit makes three breaking changes:

- Doom now fully and dynamically generates (and byte-compiles) your
  profile and its init files, which includes your autoloads, loading
  your init files and modules, and then some. This replaces
  doom-initialize-modules, doom-initialize-core-modules, and
  doom-module-loader, which have been removed. This has also improved
  startup time by a bit, but if you use these functions in your CLIs,
  for instance, this will be a breaking change.
- `doom sync` is now required for Doom to see your profiles (and must be
  run whenever you change them, or when you up/downgrade Emacs across
  major versions).
- $DOOMDIR/init.el is now read much earlier than it used to be. Before
  any of doom-{ui,keybinds,editor,projects}, before any autoloads are
  loaded, and before your load-path has been populated with your
  packages. It now runs in the context of early-init.el, giving users
  freer range over what they can affect, but a more minimalistic
  environment to do it in.

  If you must have some logic run when all that is set up, add it to one
  of the module hooks added in e08f68b or 283308a.

This also poses a significant change to Doom's load order (see the
commentary change in lib/doom.el), along with the following (non
breaking) changes:

1. Adds a new `doom profiles sync` command. This will forcibly resync
   your profiles, while `doom sync` will only do so if your profiles
   have changed.
2. Doom now fully and dynamically generates (and byte-compiles) your
   user-init-file, which includes loading all your init files, modules,
   and custom-file. This replaces the job of doom-initialize-modules,
   doom-initialize-core-modules, and doom-module-loader, which have been
   removed. This has also improved startup time by a bit.
3. Defines new doom-state-dir variable, though not used yet (saving that
   and the other breaking changes for the 3.0 release).
4. Redesigns profile directory variables (doom-profile-*-dir) to prepare
   for future XDG-compliance.
5. Removed unused/unimportant profile variables in doom.el.
6. Added lisp/doom-profiles.el. It's hardly feature complete, but it's
   enough to power the system as it is now.
7. Updates the "load order" commentary in doom.el to reflect these
   changes.

docs/examples.org

@@ -714,11 +714,92 @@ These are side-by-side comparisons, showing how to bind keys with and without
 **** TODO with-file-contents!
 
 ** TODO Configuration files
-*** TODO doomprofiles.el
+*** =profiles.el=
+:PROPERTIES:
+:ID:       f9bce7da-d155-4727-9b6f-b566b5b8d824
+:END:
+This file can live in any of:
+
+- =$DOOMDIR/profiles.el=
+- =$EMACSDIR/profiles.el=
+- =~/.config/doom-profiles.el=
+- =~/.doom-profiles.el=
+
+Here is an exhaustive example of all its syntax and capabilities:
+#+begin_src emacs-lisp
+;; -*- mode: emacs-lisp; -*-
+((profile1
+  ;; The permitted formats of each entry:
+  (var . value)
+  ("envvar" . value)
+  (var :directive values...)
+
+  ;; `user-emacs-directory' is often the first variable you want to set, so
+  ;; Emacs knows where this profile lives. If you don't, it'll use the config
+  ;; living in the default locations (~/.config/emacs or ~/.emacs.d).
+  (user-emacs-directory . "~/another/emacs/config/")
+  ;; If this is a Doom config, you'll also want to set `doom-user-dir', which
+  ;; defaults to ~/.config/doom or ~/.doom.d:
+  (doom-user-dir . "~/another/doom/config/")
+  ;; If a CAR is a string, it is assumed you want to set an environment
+  ;; variable. (Side-note: setting DOOMDIR will be unnecessary if you're setting
+  ;; `doom-user-dir' above).
+  ("DOOMDIR" . "~/another/doom/config/")
+
+  ;; Doom profiles support a number of special directives. They are:
+  ;;
+  ;; (VAR :path SEGMENTS...) -- set VAR to an exapnded path built from SEGMENTS,
+  ;; relative to `user-emacs-directory', unless an absolute path is in SEGMENTS.
+  (doom-cache-dir :path doom-user-dir ".local/cache")
+  (doom-data-dir  :path doom-user-dir ".local/data")
+  (doom-state-dir :path doom-user-dir ".local/state")
+  ;; (VAR :plist VALUE) -- use VALUE as a literal plist; ignoring any profile
+  ;; directives that may be in it.
+  (some-plist :plist (:foo bar :baz womp))
+  ;; (VAR :eval FORMS...) -- use to evaluate arbitrary elisp forms. Note that
+  ;; his runs early in early-init.el. It's wise to assume no APIs are available
+  ;; or loaded, only the previous bindings in this profile.
+  (doom-theme :eval (if (equal (system-name) "foo") 'doom-one 'doom-dracula))
+  ;; Though discouraged, you may evaluate forms without a binding by using `_'.
+  ;; You really should be doing this in the profile though...
+  (_ :eval (message "Hello world!"))
+  (_ :eval (with-eval-after-load 'company (setq-default company-idle-delay 2.0)))
+  ;; (VAR :prepend FORMS...) or (VAR :append FORMS...) -- prepend or append the
+  ;; evaluated result of each form in FORMS to VAR (a list). If VAR is undefined
+  ;; at startup, it will be deferred until the variable is available.
+  (load-path :prepend (expand-file-name "packages/" doom-user-dir))
+  (load-path :prepend (expand-file-name "lisp/" doom-user-dir))
+  (load-path :append  (expand-file-name "fallback/" doom-user-dir))
+  (exec-path :prepend (expand-file-name "bin/" doom-user-dir))
+  (auto-mode-alist :prepend '("\\.el\\'" . lisp-mode)))
+
+ (profile2
+  ...)
+
+ (profile3
+  ...))
+#+end_src
+  
+*** =.doomprofile=
+:PROPERTIES:
+:ID:       ac37ac6f-6082-4c34-b98c-962bc1e528c9
+:END:
+This file takes after the second level of =profiles.el='s format (see a more
+complete example in [[id:f9bce7da-d155-4727-9b6f-b566b5b8d824][the previous section]]). For example:
+
+#+begin_src emacs-lisp
+;;; -*- mode: emacs-lisp -*-
+;; A .doomprofile can be placed under an implicit profile. Same rules as
+;; .doom-profiles.el, but one level deeper.
+
+((var . value)
+ ("envvar" . value)
+ (var :directive values...))
+#+end_src
+
 *** TODO =.doomrc=
 *** TODO =.doomproject=
 *** TODO =.doommodule=
-*** TODO =.doomprofile=
 ** TODO Templates
 *** TODO User configuration
 *** TODO Module