refactor(lib): provide doom-libs as subfeatures

This allows us to load them via doom-require. Why not use normal
features? Because Doom's libraries are designed to be loaded as part of
Doom, and will openly rely on Doom state if needed; this is a contract I
want to enforce by ensuring their only entry points are through
`doom-require` or autoloading.

I will add them to the rest of the libraries later.

Site-node: this also adds Commentary+Code to the comment headings, as I
want a space to use that space to describe the library, when I get
around to it.

lisp/lib/plist.el

@@ -1,4 +1,6 @@
 ;;; lisp/lib/plist.el -*- lexical-binding: t; -*-
+;;; Commentary:
+;;; Code:
 
 ;;
 ;;; Macros
@@ -59,3 +61,6 @@
       (push (cadr plist) keys)
       (setq plist (cddr plist)))
     keys))
+
+(provide 'doom-lib '(plist))
+;;; plist.el ends here