feat: Keep eln-cache out of my config directory.

For early-init reasons, we had to split part of syd-prelude.el out into a new
file, syd-constants.el.

users/crumb/programs/emacs/lib/syd-constants.el

@@ -0,0 +1,13 @@
+;;; syd-constants.el -*- lexical-binding: t; -*-
+
+(defvar syd-data-dir
+  (or (getenv "EMACS_DATA_DIR")
+      (error "Need $EMACS_DATA_DIR"))
+  "Directory analogous to XDG_DATA_HOME for miscellaneous Emacs things.")
+
+(defvar syd-cache-dir
+  (or (getenv "EMACS_CACHE_DIR")
+      (error "Need $EMACS_CACHE_DIR")))
+
+(provide 'syd-constants)
+;;; syd-constants.el ends here