Fix non-development package installation

[berquist]

This depends on #80. It installs the schema files, and CI is set up to test the installed package. In order for this to work, the tests needed to be installed alongside the rest of the package, which is part of cookiecutter "best practices" anyway.

[berquist]

Without specifying package_data, this works fine with pip in an old-style venv, but not in a conda env, still using pip. It seems independent of setuptools version.

[berquist]

I cannot figure out why this is passing in GHA but not locally for me, even inside of different Docker containers.