OSS nblint fixes

Install and usage:
$ python3 -m pip install -U [--user] git+https://github.com/tensorflow/docs
$ python3 -m tensorflow_docs.tools.nblint [options] notebook.ipynb

PiperOrigin-RevId: 313595873

Author: lamberta

setup.py

@@ -32,6 +32,7 @@
 REQUIRED_PKGS = [
     'astor',
     'absl-py',
+    'protobuf',
     'pyyaml',
 ]
 

tools/nblint/__init__.py renamed to tools/tensorflow_docs/tools/__init__.py

@@ -17,7 +17,7 @@
 r"""Check notebook conformance with documentation styles.
 
 Install the tensorflow-docs package:
-$ pip3 install -U [--user] git+https://github.com/tensorflow/docs
+$ python3 -m pip install -U [--user] git+https://github.com/tensorflow/docs
 
 Usage:
 $ python3 -m tensorflow_docs.tools.nblint [options] notebook.ipynb [...]

tools/nblint/style/__init__.py renamed to tools/tensorflow_docs/tools/nblint/__init__.py

@@ -0,0 +1,14 @@
+# Copyright 2020 The TensorFlow 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.
+# ==============================================================================