Schema: Move revision directory target to a new type

Add a target layer between the revision and the target directory nodes

The reason for these intermediate target objets is to handle missing reference errors and to expose a target SWHID before retieving it from the archive.

earlier the strcuture was

Revision {
  swhid
  ..
  directory {
    swhid
  } 
}

With this change that will be

Revision {
  swhid
  ..
  directory {
    swhid
    node {
      swhid
      ..
    }
 }