Fixed the import issue (#2158)

* Fixed the import issue

* make black happy

* Use find_namespace_packages

ml-agents/setup.py

-from setuptools import setup, find_packages
+from setuptools import setup, find_namespace_packages
 from os import path
 from io import open
 
         "License :: OSI Approved :: Apache Software License",
         "Programming Language :: Python :: 3.6",
     ],
-    packages=["mlagents.trainers"],  # Required
+    # find_namespace_packages will recurse through the directories and find all the packages
+    packages=find_namespace_packages(
+        exclude=["*.tests", "*.tests.*", "tests.*", "tests"]
+    ),
     zip_safe=False,
     install_requires=[
         "mlagents_envs==0.8.2",