This will allow us to express intermediate values of
`hashDerivationModulo` in the following commit.

Use the newly parameterized derivation types to enforce the invariants
needed to split these.

…mplate

…son2314/nix into derivation-template

…/nix into derivation-template-temp

…-include-order

This further continues with the dependency inverstion. Also I just went
ahead and exposed `parseDerivation`: it seems like the more proper
building block, and not a bad thing to expose if we are trying to be
less wedded to drv files on disk anywas.

…-include-order

…emplate

…mplate

… into derivation-template

…-include-order

…include-order

This assumption is broken by CA derivations. Making a PR now to do the
breaking daemon change as soon as possible (if it is already too late,
we can bump protocol intead).

…low-missing

…-include-order

…llow-missing

…llow-missing

…llow-missing

Path is null when not known statically.

…emplate

…on-template

…into just-derivation-template

… into derivation-template

See comment for details.

…314/nix into HEAD

The name is now stored with the derivation itself.

…ems/nix into single-ca-drv-build

…into derivation-template

…on-template

Since the jsonObject unique ptr is reset to flush the string to make
`__json`, all these `!jsonObject` conditions will always be true.

…tems/nix into derivation-template

This is much better than making the caller responsible.

Now covers derivations, with and without input derivations, and all
possible `hashModuloDerivation`-motivated template parameters.

See store-api.hh new method description for details.