We had to rename SlicerPython to PythonSlicer, because some Python IDEs (specifically PyCharm, but potentially others) only recognized Python*.exe files as Python interpreters. We have not removed SlicerPython for backward compatibility but eventually it will go away (see related issue).
My understanding is that regardless of which launcher (Slicer.exe or PythonSlicer.exe) executes python-real.exe, extensions should be added to path/pythonpath. However, this does not seem to be the case. @jcfr do you know why?
Slicer modules will only be available if you execute SlicerApp-real.exe.