build: use find_package(Python3) rather than PYTHON_HOME

The behaviour of `PYTHON_HOME` can be emulated by setting
`Python3_EXECUTABLE` to the absolute path instead of the custom variable
now that we can find the python interpreter.

Author: compnerd

lldb/cmake/modules/LLDBConfig.cmake

@@ -148,6 +148,7 @@ if (LLDB_ENABLE_PYTHON)
     include_directories(${Python3_INCLUDE_DIRS})
 
     if (NOT LLDB_RELOCATABLE_PYTHON)
+      get_filename_component(PYTHON_HOME "${Python3_EXECUTABLE}" DIRECTORY)
       file(TO_CMAKE_PATH "${PYTHON_HOME}" LLDB_PYTHON_HOME)
     endif()
   else()