Support CUDA v6/CUDSS v0.7 and use cuSOLVER as dependency instead of CUDA

[ytdHuang]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

This PR directly set cuSOLVER as dependency for the extension instead of the entire CUDA package.

It still works if users load CUDA v6, since it internally loads cuSOLVER, too.

This PR also includes the changes in PR #983

[ytdHuang]

Note that the admins haven't decide whether to drop the extension of CUSOLVERRF.jl or not yet.

So currently, I only remove CUSOLVERRF in test/gpu/Project.toml