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/system.el

@@ -106,3 +106,6 @@ Tries to be portable. Returns 1 if cannot be determined."
                   (user-error "Failed to look up number of processors, because:\n\n%s"
                               (cdr cpus)))))
              1)))))
+
+(provide 'doom-lib '(system))
+;;; system.el ends here