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
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127459882

java/google/registry/BUILD

@@ -8,6 +8,7 @@ package_group(
     packages = [
         "//java/google/registry/...",
         "//javatests/google/registry/...",
+        "//python/...",
     ],
 )