Support non-poetry build standard

[waarmond]

I'm trying to update the Arch pkg instructions.

python3 -m poetry build

• fills ~/.cache/pypoetry/virtualenvs
• mirrors system includes of {boost,cvodes,Eigen,idas,kinsol,nvector,pybind11,rapidjson,stan,sundials,sunlinsol,sunmatrix,sunmemory,sunnonlinsol,tbb}

which is both undesired.

May you please support Python internal build tools?

make
python -m build --wheel --no-isolation

already builds, but doesn't pack a wheel file for

python -m installer --destdir="$pkgdir" dist/*.whl

[riddell-stan]

Moving away from poetry is a priority. The plan is to use scikit-build, https://scikit-build.readthedocs.io/en/latest/

The first step is getting httpstan building with CMake. We're looking for help with this!