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/rfcToKindle/default.nix

@@ -0,0 +1,13 @@
+{
+  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 = [];
+}