Add ext_modules to BUILD entry

This is going to be necessary to release Py3 with abi3.

The line however results in the issue that this PR is going to aim to fix: now Python 2 will be built with abi `cp27m` or `cp27mu`, whereas earlier it was `none`. To test, try running `./pants setup-py --run="bdist_wheel --python-tag cp27 --plat-name=linux_x86_64" src/python/pants:pants-packaged` followed by `ls -l dist/pantsbuild.pants-1.14.0rc0/dist/`.

Note also that ext_modules is deprecated in favor of distutils.Extension. We use this now as a temporary workaround until we add support for Extension.

src/python/pants/BUILD

@@ -18,6 +18,7 @@ python_library(
     name='pantsbuild.pants',
     description='A scalable build tool for large, complex, heterogeneous repos.',
     namespace_packages=['pants', 'pants.backend'],
+    ext_modules=[('native_engine', {'sources': []})],
   ).with_binaries(
     pants='src/python/pants/bin:pants',
   )