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

tools/url-blocker/shell.nix

@@ -1,12 +1,10 @@
 let
-  pkgs = import <unstable> {};
-in pkgs.mkShell {
-  buildInputs = with pkgs; [
-    (haskellPackages.ghcWithPackages (hpkgs: with hpkgs; [
-      time
-      aeson
-      either
-      hspec
-    ]))
+  briefcase = import <briefcase> {};
+in briefcase.buildHaskell.shell {
+  deps = hpkgs: with hpkgs; [
+    time
+    aeson
+    either
+    hspec
   ];
 }