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

rfcToKindle/default.nix

@@ -1,13 +0,0 @@
-{
-  depot ? import <depot> {}, ...
-}:
-
-# TODO: This doesn't depend on `sendgmr` at the moment, but it should. As such,
-# it's an imcomplete packaging.
-depot.buildGo.program {
-  name = "rfcToKindle";
-  srcs = [
-    ./main.go
-  ];
-  deps = [];
-}