[lldb] Get rid of helper CMake variables for Python

This patch is a big sed to rename the following variables:

  s/PYTHON_LIBRARIES/Python3_LIBRARIES/g
  s/PYTHON_INCLUDE_DIRS/Python3_INCLUDE_DIRS/g
  s/PYTHON_EXECUTABLE/Python3_EXECUTABLE/g
  s/PYTHON_RPATH/Python3_RPATH/g

I've also renamed the CMake module to better express its purpose and for
consistency with FindLuaAndSwig.

Differential revision: https://reviews.llvm.org/D85976