[lldb] Move PassthroughScriptedProcess to `lldb.scripted_process` module

This patch moves the `PassthroughScriptedProcess` & `PassthroughScriptedThread`
classes from the `interactive_scripted_process.py` test implementation
to the `lldb.scripted_process` python module.

This class is very versatile so it makes more sense to ship it with the
python module to make it easier for our adopters to derive their class
from it instead of copying it.

During the "migration", I've also noticed some bugs in the
`PassthroughScriptedThread` creation and update, so I also fixed that as
part of this patch.

Differential Revision: https://reviews.llvm.org/D151044



Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>