FindPVPythonEnvironment: check for pvpython-launcher

On some operating systems and in certain configurations, ParaView creates a launcher for each of its executables. The launcher is responsible for setting up the appropriate environment and then launching the real executable. When a launcher is not available, it is likely because the rpath for the real executable has been set to accomplish the task of configuring the environment. We first check if ParaView has a launcher for pvpython, and fall back to using the real pvpython.