This subcommand is really under-maintained (e.g. doesn't support
non-CUDA backends), nobody really seems to use it, and wheels bring back
the issues that `kernels` is supposed to solve.

Let's instead focus on getting more adoption and testing of kernel
version locking/pre-downloading.

This change adds support for backend-specific dependencies. We use the dependencies from `kernel-builder` and validate upon loading that all dependencies are permitted dependencies.