Move to using package_data to install typeshed and xml files.

[emmatyping]

Currently, we use data_files to distribute typeshed and xml files. The prefix under which data_files is installed is not exposed publicly, which has lead to complexities and issues with resolving typeshed's install location such as #4142.

Therefore, I propose we move to package_data. This will install typeshed and the xml files into site-packages/mypy/typeshed and site-packages/mypy/xml. This will be much easier to find, and should avoid all of the intricacies in resolving the system prefix.

[emmatyping]

Well, annoyingly, it appears this won't be possible, unless we require pip install . to use admin on Windows (see pypa/pip#1006).

However, python3 setup.py install will work fine, as well as sdist, bdist_wheel, and just about any other installation I can think of (including oddly pip install -e ., which is a developer install). The underlying issue is typeshed symlinks, so we could fix that and everything would work, but Im not sure the path forward on that is the clearest to me.

[JelleZijlstra]

Wasn't that issue fixed in pypa/pip#1311?

[emmatyping]

It was fixed, but it seems that the fix requires administrator access on Windows see pypa/pip#1311 (comment)

I tested it myself and it fails without privileges.

[JukkaL]

I think that we should get rid of typeshed symlinks.

[emmatyping]

Closing in favor of #5307