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

deepmind/part_one/which-appears-twice.py

@@ -1,29 +0,0 @@
-import unittest
-
-
-def find_repeat(xs):
-    n = max(xs)
-    expected_sum = (n + 1) * n / 2
-    actual_sum = sum(xs)
-    return actual_sum - expected_sum
-
-
-# Tests
-class Test(unittest.TestCase):
-    def test_short_list(self):
-        actual = find_repeat([1, 2, 1])
-        expected = 1
-        self.assertEqual(actual, expected)
-
-    def test_medium_list(self):
-        actual = find_repeat([4, 1, 3, 4, 2])
-        expected = 4
-        self.assertEqual(actual, expected)
-
-    def test_long_list(self):
-        actual = find_repeat([1, 5, 9, 7, 2, 6, 3, 8, 2, 4])
-        expected = 2
-        self.assertEqual(actual, expected)
-
-
-unittest.main(verbosity=2)