Remove packages, package_dir once setuptools>=61

[jacobdadams]

Starting with setuptools 61.0.0, it will auto-detect packages using the standard src-tree structure. However, ArcGIS Pro 3.0 still includes 58.0.4. Once the default Pro install has a compatible version of setuptools, I vote we drop the packages and package_dir keywords from setupy.py.

[steveoh]

With the new pro version out we should see if they bumped it.