[FLINK-37396][build] Use multiple top-level packages discovered in a flat-layout in setup.py

This closes  #157.

Author: loserwang1024

flink-python/setup.py

@@ -133,6 +133,7 @@ def prepare_pyflink_dir():
 
 # Runs the python setup
 setup(
+    py_modules=[],
     name=PACKAGE_NAME,
     version=connector_version,
     include_package_data=True,