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/wpc-python.el

@@ -3,7 +3,6 @@
 ;; Author: William Carroll <wpcarro@gmail.com>
 ;; Version: 0.0.1
 ;; Package-Requires: ((emacs "24"))
-;; Homepage: https://user.git.corp.google.com/wpcarro/briefcase
 
 ;;; Commentary:
 ;; My Python configuration settings