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/deepmind/part_two/.envrc

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

scratch/deepmind/part_two/dir-locals.nix

@@ -1,3 +0,0 @@
-let
-  briefcase = import <briefcase> {};
-in briefcase.utils.nixBufferFromShell ./shell.nix

scratch/deepmind/part_two/shell.nix

@@ -1,5 +1,6 @@
 let
-  pkgs = import <nixpkgs> {};
+  briefcase = import <briefcase> {};
+  pkgs = briefcase.third_party.pkgs;
 in pkgs.mkShell {
   buildInputs = with pkgs; [
     nodejs

scratch/groceries/.envrc

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

scratch/groceries/shell.nix

@@ -1,8 +1,5 @@
 let
-  pkgs = import <unstable> {};
-in pkgs.mkShell {
-  buildInputs = with pkgs; [
-    (haskellPackages.ghcWithPackages (hpkgs: with hpkgs; [
-    ]))
-  ];
+  briefcase = import <briefcase> {};
+in briefcase.buildHaskell.shell {
+  deps = hpkgs: [];
 }

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
   ];
 }