Discriminate test-packages and host-packages when building tests.
This avoids the test tooling being tainted with the hostPlatform
and targetPlatform of the test. For example, QEMU must be built
independently and isolated from the guest platform, for the sake
of test control and minimizing rebuilds.
We need the latest version of the Dhall interpreter, but that needs
a pin to Nixpkgs release-17.09. Use submodule rather than a fetchgit
because dhall-haskell requires a submodule itself, and fetgit does
not pull this in.