Support building from PyPI

[karosc]

For the longest time, building this project from source required cloning from git, pulling the submodule, and running the setup.py script.

Currently, if a user tries to pip install swmm-toolkit, and a wheel for their given version of python or system is not available on PyPI, pip tries to build from the source dist uploaded to PyPI and fails because the build tools are not installed. This PR attempts to resolve that problem by adding a pyproject.toml file that specifies the build backend and build tools.

In addition to the pyproject.toml, I had to add a small hack to the setup.py script to trick setuptools. I had to add this because scikit-build and setuptools don't play nice with pip install . or python -m build. When running code that generates the sdist or dist_info data, setuptools checks that packages are available. Since our package is only built when running cmake (from a different dir too), those package directories or not found because cmake is not run for sdist or dist_info, and the validation fails. Changing the setuptools package_dir parameter depending on if cmake is running gets around the issue, and I was able to run pip install . and python -m build . successfully in blank virtual envs.

Eventually it'd be nice to transition our build backend to scikit-build-core, but I have not found a good way to make it work yet for multi-platform builds.