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/find-duplicate-optimize-for-space-beast.py

@@ -0,0 +1,56 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def find_duplicate(xs):
+    self_ref_count = 0
+    for i in range(len(xs)):
+        if xs[i] == i + 1:
+            self_ref_count += 1
+    hops = len(xs) - 1 - self_ref_count
+    current = xs[-1]
+    while hops > 0:
+        current = xs[current - 1]
+        hops -= 1
+    return current
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    # TODO: Debug why this fails.
+    def test_darren_from_interview_cake(self):
+        actual = find_duplicate([4, 1, 8, 3, 2, 7, 6, 5, 4])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_just_the_repeated_number(self):
+        actual = find_duplicate([1, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_short_list(self):
+        actual = find_duplicate([1, 2, 3, 2])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_last_cycle(self):
+        actual = find_duplicate([3, 4, 2, 3, 1, 5])
+        expected = 3
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_duplicate([1, 2, 5, 5, 5, 5])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_duplicate([4, 1, 4, 8, 3, 2, 7, 6, 5])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)