Reduce configuration done in `early-init.el`

As per the documentation (see "(elisp)Startup Summary"), only variable
customization that affect package initialization should go into the early init
file.  Defining package archives is explicitly mentioned as something that still
may go into the main init file.  So we move it there and adjust comments
accordingly.

early-init.el

@@ -1,11 +1,5 @@
 ;; -*- lexical-binding: t -*-
 
-(require 'package)
-
 (setq package-user-dir (expand-file-name "elpa" user-emacs-directory))
 
-(setq package-archives
-      '(("melpa" . "https://melpa.org/packages/")
-        ("gnu" . "https://elpa.gnu.org/packages/")
-        ("melpa-stable" . "https://stable.melpa.org/packages/")
-        ("org" . "https://orgmode.org/elpa/")))
+

init.el

@@ -42,9 +42,15 @@
 
 ;; * Packages
 
+(setq package-archives
+      '(("melpa" . "https://melpa.org/packages/")
+        ("gnu" . "https://elpa.gnu.org/packages/")
+        ("melpa-stable" . "https://stable.melpa.org/packages/")
+        ("org" . "https://orgmode.org/elpa/")))
+
 (when (< emacs-major-version 27)
   ;; Before Emacs 27.1, we had to do package initialization ourselves.  In Emacs
-  ;; 27.1 and later, it's done in early-init.el.  See
+  ;; 27.1 and later, it's done directly after loading early-init.el.  See
   ;; https://www.gnu.org/software/emacs/news/NEWS.27.1
   (load-file (expand-file-name "early-init.el" emacs-d))
   (package-initialize))