Using sdist to package root source files that are in a subdirectory

Question

I have the following file layout:

setup.py
build/
  generated-sources/
    foo.py

With the following setup script:

setup(
    package_dir={'': 'build/generated-sources'},
    packages=[''])

From the docs (https://docs.python.org/2/distutils/examples.html) I was expecting the package to contain just foo.py at the root level (since it's referenced as the root package ''), but the tar file contains the intermediate directories (i.e., build/generated-sources/foo.py)

How can I create a package that just has foo.py at the root?