[lldb] Unifying lldb python path

Based on mgorny@'s D67890

There are 3 places where python site-package path is calculated
independently:

1. finishSwigPythonLLDB.py where files are written to site-packages.

2. lldb/scripts/CMakeLists.txt where site-packages are installed.

3. 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 compiling users to specify the right
path for site-packages.

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68442

llvm-svn: 373991