Generated files must be present before invocation to be included in package_data

I've created the following repository to demonstrate the problem:
https://github.com/JonathonReinhart/setuptools-package_data-issue

Copying what I've shown there:

This repository is an MCVE for the following issues:

• https://github.com/JonathonReinhart/scuba/issues/77
• https://github.com/JonathonReinhart/staticx/issues/22

Problem statement

When python setup.py sdist bdist_wheel is invoked from a clean starting point, data files (identified in package_data), which are generated during setup.py hooks, will not be included in the wheel.

• From a clean starting point (git clean -fdx) (primarily removes mkpkg/generated_data):
  • python setup.py bdist_wheel - OK
  • python setup.py sdist then python setup.py bdist_wheel - OK
  • python setup.py sdist bdist_wheel - BROKEN

Steps to reproduce

First:

• Run git clean -fdx -- remove any generated files
• Run python setup.py sdist bdist_wheel
• Observe only the following line in the output:
  • copying build/lib/mypkg/__init__.py -> build/bdist.linux-x86_64/wheel/mypkg
• Run unzip -l dist/mypkg-1.2.3-py2-none-any.whl
  • Observe that mypkg/generated_data is missing from the wheel

Now that mypkg/generated_data is present:

• Run python setup.py sdist bdist_wheel again
• Observe the following lines in the output:
  • copying build/lib/mypkg/generated_data -> build/bdist.linux-x86_64/wheel/mypkg
  • copying build/lib/mypkg/__init__.py -> build/bdist.linux-x86_64/wheel/mypkg
• Run unzip -l dist/mypkg-1.2.3-py2-none-any.whl
  • Observe that mypkg/generated_data is now present in the wheel

Clean the directory, and run other variations of sdist and bdist_wheel (as mentioned above) and observe that the problem does not manifest.

Workarounds that do not work

• Hooking build_py instead of build

Start by reproducing the issue with git clean -fdx and python setup.py sdist bdist_wheel, then inspect setup.py, its generation hooks, package_data, sdist, and bdist_wheel behavior. Compare the combined command with the separate sdist and bdist_wheel runs. Done means generated_data is included in the wheel from a clean starting point.