[SPARK-3634] [PySpark] User's module should take precedence over system modules

Python modules added through addPyFile should take precedence over system modules.

This patch put the path for user added module in the front of sys.path (just after ''), 

BTW: it's a bit dangerous that user can upload new module to modify the default behavior of system. Currently, it's hard to find the the correct position to insert user's module.