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

default.nix

@@ -23,6 +23,7 @@ let
     gopkgs = readTree ./gopkgs;
     monzo_ynab = readTree ./monzo_ynab;
     third_party = readTree ./third_party;
+    tools = readTree ./tools;
   };
 in fix(self: {
   config = config self;