Clarify the method of distribution for stub files with extension only packages.

PEP-0561 does not mention extension packages, packages that consist of just an .so file. Given that PEP-484 mentions extension modules as a use case for stub files, could the typing documentation make it more clear what the ideal layout of an extension only package is?
It appears to be valid to have a package structure like the following:

.../site-packages/
    mypackage/
        __init__.pyi
        py.typed
    mypackage.cpython-37m-x86_64-linux-gnu.so

@ethanhs suggested in https://github.com/python/peps/issues/2318 that this is the correct way of doing things.

首先阅读 issue 中的 PEP-0561 和 PEP-484 参考资料，然后查看有关包分发和存根文件的 typing 文档。明确所示的仅包含扩展的包布局是否受支持，并记录理想的布局，包括 init.pyi 和 py.typed 的放置位置。