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

haskell-file/.envrc

@@ -0,0 +1,2 @@
+source_up
+use_nix

haskell-file/shell.nix

@@ -1,9 +1,5 @@
-with import <nixpkgs> {};
-
-stdenv.mkDerivation {
-  name = "f-hs";
-  buildInputs = [
-    (pkgs.haskellPackages.ghcWithPackages (pkgs: [
-    ]))
-  ];
+let
+  briefcase = import <briefcase> {};
+in briefcase.buildHaskell.shell {
+  deps = hpkgs: [];
 }