Disable auto-discovery for setuptools

Since setuptools release (61.0.0) ansible-collection-openstack's
package build command (python3 setup.py sdist bdist_wheel) is
finding multiple top-level packages in a flat-layout automatically.

This issue is mentioned in setuptools bug 3197 [1], and the suggested
workaround is to disable auto-discovery by adding 'py_modules=[]' in
setup.py.

[1] https://github.com/pypa/setuptools/issues/3197

Change-Id: I4aef1fd59375c4a3bc9e362e7949fa153e4cbcb0

setup.py

@@ -5,4 +5,5 @@ import setuptools
 
 setuptools.setup(
     setup_requires=['pbr'],
-    pbr=True)
+    pbr=True,
+    py_modules=[])