Tidy up structure of briefcase

I had a spare fifteen minutes and decided that I should tidy up my
monorepo. The work of tidying up is not finished; this is a small step in the
right direction.

TL;DR
- Created a tools directory
- Created a scratch directory (see README.md for more information)
- Added README.md to third_party
- Renamed delete_dotfile_symlinks -> symlinkManager
- Packaged symlinkManager as an executable symlink-mgr using buildGo

scratch/deepmind/part_two/shell.nix

@@ -0,0 +1,10 @@
+{ pkgs ? import <nixpkgs> {}, ... }:
+
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    nodejs
+    python3
+    go
+    goimports
+  ];
+}