Refactor some elisp out of doom.nix

Just for readability, no functional change.

build-helpers/print-deps.el

@@ -0,0 +1,14 @@
+(require 'package)
+
+(with-temp-buffer
+  (setq default-directory (car command-line-args-left))
+  (dired-mode)
+  ;; Ignore dependency extraction errors because it fails for repos not
+  ;; containing a "proper" package (no -pkg.el, no file with the right magic
+  ;; header). These seem common enough to be not worth allowlisting.
+  (let ((reqs (with-demoted-errors "Extracting dependencies: %s"
+                (package-desc-reqs (package-dir-info)))))
+    (princ
+     (json-encode
+      (mapcar #'car (seq-remove (lambda (p) (apply #'package-built-in-p p))
+                                reqs))))))