Add top-level python directory to opensource repo

This adds a home in our opensource repo for python libraries and binaries,
under a top-level "python" directory.  Future CLs will relocate ICANN
reporting bits and pieces to new homes under this directory, and will use
the MOE configuration and python_directory_import rule defined here.

This approach is roughly modeled on the protobuf Bazel opensource project,
which also uses a top-level directory for various languages, and also uses
the "imports" parameter to exclude that directory in python module names:
  https://github.com/google/protobuf/blob/v3.0.0-beta-3/BUILD#L568
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127459882

python/BUILD

@@ -0,0 +1,27 @@
+# Top-level directory for python code in the domain registry project.
+#
+# Code within this directory should live under python/google/registry.
+#
+# For an example of another Google open-source Bazel-based project using a
+# top-level python directory in a similar way, see
+#   https://github.com/google/protobuf/tree/master/python
+
+package(default_visibility = ["//java/google/registry:registry_project"])
+
+licenses(["notice"])  # Apache 2.0
+
+
+# NOTE: All py_library() rules under this directory should depend on this rule.
+#
+# This rule serves to add this directory (/python) to the PYTHONPATH of any
+# python executable rules that transitively depend on this rule.  Adding this
+# directory ensures that imports for "google.registry.*" can be correctly
+# resolved to files that are under //python/google/registry.
+#
+# See also:
+#   - http://www.bazel.io/docs/be/python.html#py_library.imports
+#   - https://github.com/google/protobuf/blob/v3.0.0-beta-3/BUILD#L568
+py_library(
+    name = "python_directory_import",
+    imports = ["."],
+)