Allow namespace references in the playground

[mjkoster]

We need to be able to host SDF files with namespace references, either in the playground or in another repository.

References of this sort:

"odmRef": "zcl:/odmData/TransitionTimeData" where there was another file already in the repo that defines #/odmData/TransitionTimeData.

2 parts:

1. Language definition, which we mostly agreed on during the the file roll-up discussion we had at the last F2F
2. Processing in the playground to resolve references to files already in the space