Expects storage driver executables to be of the form registry-storage-<name>

Also moves main packages to sub-packages of the individual storage drivers

This should make finding storage drivers on the executable $PATH more sane while we decide on a potential plugin/driver directory structure.
IMHO, we should go with the `$PATH` approach. This is pretty familiar in that its used with git and protobuf compiler for binary plugin implementations. It can be disorienting for the uninitiated, but we can cover that with documentation.