pw_build: Generate setup.cfg files

Modify generated_python_package to create setup.cfg files instead of
setup.py files. This will make for easier parsing of package data by
other build tooling.

pw_python_package's generate_setup option has been updated to follow
setup.cfg structure. Relevant sections should be nested in the correct
scope. For example here is the old format:

generate_setup = {
  name = "a_nifty_package"
  version = "1.2a"
  install_requires = [ "a_pip_package" ]
}

New format:

generate_setup = {
  metadata = {
    name = "a_nifty_package"
    version = "1.2a"
  }
  options = {
    install_requires = [ "a_pip_package" ]
  }
}

This will generate a setup.cfg similar to the example here:
https://setuptools.readthedocs.io/en/latest/userguide/declarative_config.html

Change-Id: I25fd7503ecc2d768d385bbb940a2715f0739c03b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/56960
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Anthony DiGirolamo <tonymd@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>