- Pass it the name of the outputs rather than their output paths (as
  these don't exist for ca derivations)
- Get the built output paths from the remote builder
- Register the new received realisations

To allow it to build ca derivations remotely

To allow it to build ca derivations remotely

To allow it to build ca derivations remotely

Otherwise they don't get registered, triggering an assertion failure
at some point later

It's possible that all the paths are already there, but just not
associated to the current drv output

There was already some logic for that, but it didn't handle the case of
content-addressed outputs, so extend it a bit for that