Drop support for dir-locals.nix, <nixpkgs>, etc.

In the spirit of Marie Kondo, I'm tidying up!

TL;DR:
- Prefer .envrc `use_nix` and delete all dir-locals.nix files
- Remove ~all references to <nixpkgs>, <unstable>, <depot> and prefer
  referencing each with briefcase.third_party.{pkgs,unstable,depot}
- Delete nixBufferFromShell function since I was only using that in
  dir-locals.nix files

emacs/.emacs.d/snippets/nix-mode/shell-nix

@@ -2,9 +2,10 @@
 # name: shell.nix boilerplate
 # key: import
 # --
-with import <nixpkgs> {};
-
-stdenv.mkDerivation {
+let
+  briefcase = with import <briefcase> {};
+  pkgs = briefcase.third_party.pkgs;
+in stdenv.mkDerivation {
   name = "$1";
   buildInputs = [
     $2