Building wheel suitable for extensions

Me and @berendkleinhaneveld were investigating if we could build extensions against this package. It seems to be a runtime-only package, which is great, but we need the development files, such as headers and the wrapPython executable.

It looks like overriding the option `-DVTK_INSTALL_NO_DEVELOPMENT:BOOL=ON` to `OFF` should do the trick. So we tried that, and more development files ended up in the wheel. However, _we are not seeing any header files!_

We've been investigating all day, but it's unclear to us what is causing this. Do you guys have any suggestions for where we should be looking?

Start by tracing the repository's setup script and the VTK_INSTALL_NO_DEVELOPMENT option to see how wheel contents are selected. Verify whether the generated wheel contains the development headers and wrapPython executable; done means those extension-build files are present and usable.

Written by the indexing model from the issue text.