I removed meta from the output in 434376f8e1 with the intention of adding it back gated by a flag, but that never happened.
Adding meta is quite a substantial increase in output size and has some non-trivial performance impact at scale, so it's best to leave it as opt-in.
The output of the evaluator should only include either the full
derivation (not yet implemented) or fields not directly accessible
from the drv such as meta.
Right now the output is a fairly arbitrary selection of fields.
