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/fixtures.py

@@ -0,0 +1,110 @@
+# Using this module to store commonly used, but annoying to create, data
+# structures for my test inputs.
+#
+# Use like:
+# from fixtures import graph_a
+
+################################################################################
+# Constants
+################################################################################
+
+edge_list = [
+    ('a', 'b'),
+    ('a', 'c'),
+    ('a', 'e'),
+    ('b', 'c'),
+    ('b', 'd'),
+    ('c', 'e'),
+    ('d', 'f'),
+    ('e', 'd'),
+    ('e', 'f'),
+]
+
+unweighted_graph = {
+    'a': {'b', 'c', 'e'},
+    'b': {'c', 'd'},
+    'c': {'e'},
+    'd': {'f'},
+    'e': {'d', 'f'},
+    'f': set(),
+}
+
+adjacencies = {
+    'a': {
+        'a': False,
+        'b': False
+    },
+    'a': [],
+    'a': [],
+    'a': [],
+    'a': [],
+    'a': [],
+    'a': [],
+}
+
+weighted_graph = {
+    'a': {(4, 'b'), (2, 'c'), (4, 'e')},
+    'b': {(5, 'c'), (10, 'd')},
+    'c': {(3, 'e')},
+    'd': {(11, 'f')},
+    'e': {(4, 'd'), (5, 'f')},
+    'f': set(),
+}
+
+# This is `weighted_graph` with each of its weighted edges "expanded".
+expanded_weights_graph = {
+    'a': ['b-1', 'c-1', 'e-1'],
+    'b-1': ['b-2'],
+    'b-2': ['b-3'],
+    'b-3': ['b'],
+    'c-1': ['c'],
+    'e-1': ['e-2'],
+    'e-2': ['e-3'],
+    'e-3': ['e'],
+    # and so on...
+}
+
+unweighted_digraph = {
+    '5': {'2', '0'},
+    '4': {'0', '1'},
+    '3': {'1'},
+    '2': {'3'},
+    '1': set(),
+    '0': set(),
+}
+
+################################################################################
+# Functions
+################################################################################
+
+
+def vertices(xs):
+    result = set()
+    for a, b in xs:
+        result.add(a)
+        result.add(b)
+    return result
+
+
+def edges_to_neighbors(xs):
+    result = {v: set() for v in vertices(xs)}
+    for a, b in xs:
+        result[a].add(b)
+    return result
+
+
+def neighbors_to_edges(xs):
+    result = []
+    for k, ys in xs.items():
+        for y in ys:
+            result.append((k, y))
+    return result
+
+
+def edges_to_adjacencies(xs):
+    return xs
+
+
+# Skipping handling adjacencies because I cannot think of a reasonable use-case
+# for it when the vertex labels are items other than integers. I can think of
+# ways of handling this, but none excite me.