Move the SystemMetadata class from the `dataone` package.

This allows us to avoid circular dependencies between `dataone` and `datapackage`.  In this architecture, the datapackage::DataPackage contains datapackage::DataObject instances, which in turn contain datapackage::SystemMetadata.  The `dataone` package will be refactored to use all three of these classes once everything is working here.
Remove references to the `dataone` package.