Rename package to zvm-validator from libvalidator0

The validator isn't just a library; it also includes a couple of command line tools for performing validation. But calling it just `validator` is a bit ambiguous. Thus, I think `zvm-validator` a good name for the package.

If this change is acceptable, this could set precedence for package naming conventions for the other ZeroVM projects (`zvm-toolchain`, `zvm-runtime`, etc.). This follows the naming conventions proposed in this thread: https://groups.google.com/forum/#!topic/zerovm/av3yspBkdgc

I'll be following up shortly with some more changes which will allow us to easily generate "latest" packages.