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

tools/run/default.nix

@@ -0,0 +1,16 @@
+{
+  pkgs ? import <nixpkgs> {},
+  depot ? import <depot> {},
+  briefcase ? import <briefcase> {},
+  ...
+}:
+
+depot.buildGo.program {
+  name = "run";
+  srcs = [
+    ./main.go
+  ];
+  deps = with briefcase.gopkgs; [
+    utils
+  ];
+}