Turn `//python/private` into a package. (#555)

python/private/BUILD

diff --git a/python/private/BUILD b/python/private/BUILD
new file mode 100644
index 0000000..90fcd3b
--- /dev/null
+++ b/python/private/BUILD
@@ -0,0 +1,41 @@
+# Copyright 2021 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+licenses(["notice"])  # Apache 2.0
+
+filegroup(
+    name = "distribution",
+    srcs = glob(["**"]),
+    visibility = ["//python:__pkg__"],
+)
+
+# Filegroup of bzl files that can be used by downstream rules for documentation generation
+# Using a filegroup rather than bzl_library to not give a transitive dependency on Skylib
+filegroup(
+    name = "bzl",
+    srcs = [
+        "reexports.bzl",
+    ],
+    visibility = ["//python:__pkg__"],
+)
+
+# Needed to define bzl_library targets for docgen. (We don't define the
+# bzl_library target here because it'd give our users a transitive dependency
+# on Skylib.)
+exports_files(
+    [
+        "reexports.bzl",
+    ],
+    visibility = ["//docs:__pkg__"],
+)