Move command plugin interface to libnixcmd

Change the `nix-build` logic for linking/printing the output paths to allow for
some outputs to be missing. This might happen when the toplevel
derivation didn't have to be built, either because all the required
outputs were already there, or because they have all been substituted.

Simple test to ensure that `nix-build && nix-collect-garbage &&
nix-build -j0` works as it should

Once a build is done, get back to the original derivation, and register
all the newly built outputs for this derivation.

This allows Nix to work properly with derivations that don't have all
their build inputs available − thus allowing garbage collection and
(once it's implemented) binary substitution

Where a `RealisedPath` is a store path with its history, meaning either
an opaque path for stuff that has been directly added to the store, or a
`Realisation` for stuff that has been built by a derivation

This is a low-level refactoring that doesn't bring anything by itself
(except a few dozen extra lines of code :/ ), but raising the
abstraction level a bit is important on a number of levels:

- Commands like `nix build` have to query for the realisations after the
  build is finished which is fragile (see
  27905f1 for example). Having them
  oprate directly at the realisation level would avoid that
- Others like `nix copy` currently operate directly on (built) store
  paths, but need a bit more information as they will need to register
  the realisations on the remote side

That way we can copy the realisations too (in addition to the store
paths themselves)