[Packaging]: Build-from-source fails with setuptools >= 61.0.0

[drewrisinger]

Description of the issue
Cirq install from source fails with setuptools==61.2.0. It succeeds with setuptools==54.2.0, likely higher versions (based on pypa/setuptools#3197, probably >=61.0.0)

Cirq version
0.14.1

Error Message

error: Multiple top-level packages discovered in a flat-layout: ['check', 'rtd_docs', 'dev_tools'].

To avoid accidental inclusion of unwanted files or directories,
setuptools will not proceed with this build.

If you are trying to create a single distribution with multiple packages
on purpose, you should not rely on automatic discovery.
Instead, consider the following options:

1. set up custom discovery (`find` directive with `include` or `exclude`)
2. use a `src-layout`
3. explicitly set `py_modules` or `packages` with a list of names

To find more information, look for "package discovery" on setuptools docs.

[pavoljuhas]

I can take care of this one.

[drewrisinger]

My patch in drewrisinger/nur-packages@aeac9e1 should fix this, it basically just prevents setup.py from finding any packages to install. I meant to submit that as a patch but forgot.

[pavoljuhas]

@drewrisinger - thank you for the tip - #5330 should in effect do the same.