Align all the worker protocol with `buildDerivation` which inlines the
realisations as one opaque json blob.
That way we don’t have to bother changing the remote store protocol
when the definition of `Realisation` changes, as long as we keep the
json backwards-compatible

Currently never used, nor set but will be useful shortly

Move the `closure` logic of `computeFSClosure` to its own (templated)
function.
That way we can reuse it for the realisations too

Only considers the closure in term of `Realisation`, ignores all the
opaque inputs.

Dunno whether that’s the nicest solution, need to think it through a bit

Otherwise registering the realisations on the remote side might fail as
it now expects a complete closure