This is based on mgorny's D67890.
There are 3 places where python site-package path is calculated independently:
- finishSwigPythonLLDB.py where files are written to site-packages.
- lldb/scripts/CMakeLists.txt where site-packages are installed.
- ScriptInterpreterPython.cpp where site-packages are added to PYTHONPATH.
This change creates the path once and use it everywhere. So that they will not go out of sync.
Also it provides a chance for cross compile users to specify the right path for site-packages.