Install headers using both headers and package_data

[isuruf]

Fixes#1067
Fixespybind/python_example#45

[wjakob]

This commit will break virtually every project that recursively includes pybind11 as a git submodule because you are changing the location of the header files. That's really a no-go for me.

[isuruf]

@wjakob, it turns out that changing the location is not needed. This is now backwards compatible.

[isuruf]

With this PR, pybind11 doesn't have to be installed before installing the downstream package and downstream packages need pybind11 only in setup_requires. See pybind/python_example#47

[wjakob]

cc @sdebionne, @ax3l
This PR removes some of the Conda-specific changes made as part of #1877. Would you mind taking a look?

[ax3l]

Thanks for the ping. I'll try this with spack as well after lunch.

[ax3l]

Hm, I do not think the new implementation helps much if we are in CMake-controlled/PYBIND11_USE_CMAKE installation mode... but otherwise okay for me if it helps with purely setup.py-centric installs.

[isuruf]

Even in PYBIND11_USE_CMAKE mode, the headers are installed in site-packages/pybind11/include inside the python installation, which duplicates the headers, but that's a small price to pay IMO to allow the different use-cases namely, cmake installs, pip installs, pep517 installs, submodules.

[ax3l]

Okay, sounds good to me.
I actually meant CMake-only mode. But I can use the USE_PYTHON_INCLUDE_DIR option for that, too I guess...
Update: just tested, works as well.

[wjakob]

@isuruf : I'll go ahead and merge this then, or did you plan to make additional changes?

[isuruf]

No. This is ready from my end.

[wjakob]

Ok -- merged!

[marscher]

Many thanks to all contributes/reviewers to fix this long outstanding issue! 👍

[SylvainCorlay]

pep517 is causing so much breakage, especially with the user isolation...

[ax3l]

But the pyproject.toml is so nice :)
Ok, enough spamming...

[isuruf]

@wjakob, any plans for a new release using this?

[xunkai55]

+1 for the release. It would help a lot :-)

[wjakob]

FYI: I've finally release v2.5.0, which includes this fix.