refactor(wpcarro): Remove briefcase references

TL;DR:
- prefer WPCARRO env-var to BRIEFCASE
- remove repository URLs from Emacs libraries
- prefer tvl-depot-path where possible
- reduce the scope of constants.el
- prune (some not all) stale CI configuration

Change-Id: I21e9130402502ec6fa2fc4b46753c890069be62d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4545
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

users/wpcarro/emacs/.emacs.d/wpc/ivy-helpers.el

@@ -2,7 +2,6 @@
 
 ;; Author: William Carroll <wpcarro@gmail.com>
 ;; Version: 0.0.1
-;; URL: https://git.wpcarro.dev/wpcarro/briefcase
 ;; Package-Requires: ((emacs "24.3"))
 
 ;;; Commentary: