Fix *_custom_setuptools_useable ITs. (#606)
Fix *_custom_setuptools_useable ITs. (#606)

Ensure the entrypoint function used to test both entrypoints and modules
explicitly tests that it's the import that works and then chains the
appropriate return / `__main__` behavior to express the result of the
test.
jsirois/pex,jsirois/pex,pantsbuild/pex,pantsbuild/pex