Make the build-hook work with ca derivations

- 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

src/libstore/realisation.hh

@@ -33,6 +33,8 @@ struct Realisation {
     GENERATE_CMP(Realisation, me->id, me->outPath);
 };
 
+typedef std::map<DrvOutput, Realisation> DrvOutputs;
+
 struct OpaquePath {
     StorePath path;