This commit includes various cleanups to Python interpreter
initialization in VTK.

vtkpython/pvtkpython executables now simply use vtkPythonInterpreter.
Previously, the code (which was added before vtkPythonInterpreter was
created) made direct Python API calls. Now, we simply defer to
vtkPythonInterpreter. That makes it possible to have just one
location to setup Python initialization logic in VTK.

Of HPC installations, Python default logic to locate prefix/exec_prefix
relative to the executable name had the potential to fail when using
custom Python builds. The new code tries to setup executable name in the
directory with the Python libraries, if possible.

The code to locate VTK's python modules (both platform dependent and
independent components) has been updated to look for them relative to
the VTK libraries, rather than the executable.

vtkpython now respects `-v` and `-vv` command line arguments to print
debugging information about paths looked up when setting up module
search paths.