Some of these files contain the magic string "DO NOT EDIT" but we do
not wish for our tests to depend upon that.

This should address the difference in how we ask users to install
grpcio-tools and the files the user ends up downloading from PyPI.

Setuptools was updated and our hacky namespace-package-chickens came
back to roost. This removes the unsupported namespace package hacks.

Setuptools was updated and our hacky namespace-package-chickens came
back to roost. This removes the unsupported namespace package hacks.

This should allow our tests to build on newer virtualenvs.

Fixes #8285, allows us to work on later versions of virtualenv without
getting problems with too-recent python packaging versions.