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/.envrc

@@ -1,8 +1,4 @@
-export BRIEFCASE="$(realpath .)"
-# I'm ensuring that $NIX_PATH is mostly empty, so that I only depend on
-# <briefcase> for now.
-# For more information on the NIX_PATH anti-pattern, see here:
-# https://nix.dev/tutorials/towards-reproducibility-pinning-nixpkgs.html#pinning-nixpkgs
-export NIX_PATH="briefcase=$BRIEFCASE";
+source_up
 export DESKTOP="zeno.lon.corp.google.com";
 export LAPTOP="seneca";
+export WPCARRO="$REPO_ROOT/users/wpcarro"