Further modernization of `cuda.core`'s build backend

[leofang]

[rparolin]

Closing as a duplicate of #605. This tracker has an empty body, no comments, and no cross-references in 15 months, while the live work in exactly this scope — draft PR #2607, the scikit-build-core spike for cuda.core — routes to #605. Keeping the scikit-build-core vs setuptools decision in one place.