add mypy.plugins into built packages (#4570)

Author: yedpodtrzitko

setup.py

@@ -100,7 +100,7 @@ def run(self):
       license='MIT License',
       platforms=['POSIX'],
       py_modules=[],
-      packages=['mypy', 'mypy.test', 'mypy.server'],
+      packages=['mypy', 'mypy.test', 'mypy.server', 'mypy.plugins'],
       entry_points={'console_scripts': ['mypy=mypy.__main__:console_entry',
                                         'stubgen=mypy.stubgen:main',
                                         'dmypy=mypy.dmypy:main',