[python] switch from _vendor.py to _vendor folder for vendored code

[iscai-msft]

Today when we vendor code in our generated packages, we generate the vendored code in a _vendor.py file. We want to switch this to a folder

There are a few steps to do here:

1. we want to move from _vendor.py to a _vendor folder. This is so we can easily start vendoring corehttp and coreidentity when we start on that work.
2. We can clean up the _vendor.py file when we split it into a module with different files.
3. Switch imports of our current vendored code so we don't have to expose all of our vendored utils in _vendor/__init__.py

This needs to be fixed before we can support #5807