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 $PATH more sane while we decide on a potential plugin/driver directory structure.
Good catch about the `$GOPATH` format. We can either try each component of the `$GOPATH` in order or simply require it to be findable in `$PATH`. Alternatively, we may want to define a directory where these executables may be found, as they aren't particularly useful for any other purpose.