[lldb/python] Fix scripted_platform python module creation

This patch should fix the creation and addition of the `scripted_platform`
python module into the `lldb.plugins` module.

Previously, we were creating the `plugins` submodule, each time with a
different source file (either `scripted_process` or `scripted_platform`).

The removes the redundant `create_python_package` call and group both
python source files toghether.

Differential Revision: https://reviews.llvm.org/D143122



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>