[lldb] Unifying lldb python path
Based on mgorny@'s D67890
There are 3 places where python site-package path is calculated
- 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
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 compiling users to specify the right
path for site-packages.
Differential Revision: https://reviews.llvm.org/D68442