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 $PATH more sane while we decide on a potential plugin/driver directory structure.
This isn't an entirely safe operation. `$GOPATH` _can_ be a list and this would append "/bin" to the end of that, making  the search path for the binary only work for the last `$GOPATH` entry.

We should just require the running process container to ensure that the plugins are in the standard executable path.