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

scratch/haskell-programming-from-first-principles/.envrc

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

scratch/haskell-programming-from-first-principles/shell.nix

@@ -1,10 +1,8 @@
 let
-  pkgs = import <unstable> {};
-in pkgs.mkShell {
-  buildInputs = with pkgs; [
-    (haskellPackages.ghcWithPackages (hpkgs: with hpkgs; [
-      hpkgs.quickcheck-simple
-      hpkgs.checkers
-    ]))
+  briefcase = import <briefcase> {};
+in briefcase.buildHaskell.shell {
+  deps = hpkgs: with hpkgs; [
+    quickcheck-simple
+    checkers
   ];
 }