This field used to be a `BasicDerivation`, but this `BasicDerivation`
was downcasted to a `Derivation` when needed (implicitely or not), so we
might as well make it a full `Derivation` and upcast it when needed.
This also allows getting rid of a weird duplication in the way we
compute the static output hashes for the derivation. We had to
do it differently and in a different place depending on whether the
derivation was a full derivation or just a basic drv, but we can now do
it unconditionally on the full derivation.
Fix#4559
That way we
1. Don't have to recompute them several times
2. Can compute them in a place where we know the type of the parent
derivation, meaning that we don't need the casting dance we had before
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
