If the project isn't declarative, who cares about it in the response? After
setting the `declfile` to an empty string, everything related to declarative-
ness is wiped out, anyways.
To further align with the API, we return custom JSON in order to display a
`visible` field rather than `hidden` -- a `PUT` request expects `visible`, while
a `GET` request returns `hidden`.
This also allows us to rename the `jobsetinputs` field to `inputs` for the same
reason: `PUT` expects `inputs`, while `GET` returns `jobsetinputs`.
This is a breaking change. Previously, packages named `packageset.foo`
would be exposed in the fake derivation channel as `packageset-foo`.
Presumably this was done to avoid needing to track attribute sets, and
to avoid the complexity. I think this now correctly handles the
complexity and properly mirrors the input expressions layout.
