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/data_structures_and_algorithms/mesh-message.gv

@@ -0,0 +1,11 @@
+strict graph {
+    Min -- {William, Jayden, Omar}
+    William -- {Min, Noam}
+    Jayden -- {Min, Amelia, Ren, Noam}
+    Adam -- {Amelia, Miguel, Sofia, Lucas}
+    Ren -- {Jayden, Omar}
+    Amelia -- {Jayden, Adam, Miguel}
+    Miguel -- {Amelia, Adam, Liam, Nathan}
+    Noam -- {Nathan, Jayden, William}
+    Omar -- {Ren, Min, Scott}
+}