Input tasks require JSON readable and writable output

[nrktkt]

I'm not sure what the intended behavior is, but the there's a little inconsistency between the documentation and the code for input tasks.
The task cheat sheet confirms that inputs do not need to be JSON readable or JSON writable, however T.input requires an implicit upickle ReadWriter for the result type.

steps to reproduce:

import mill._, scalalib._
import java.time.Instant

object baz extends JavaModule {
  def test = T.input(Instant.now)
}

Confirm the build does not compile due to

could not find implicit value for parameter rw: upickle.default.ReadWriter[java.time.Instant]

[lihaoyi]

Not sure how easy it would be to remove the ReadWriter dependency in Input, but one workaround could be to define a ReadWriter for java.time.Instant. Since it's in the standard library, perhaps even push it upstream to the uPickle repo

[nrktkt]

The workaround I'm currently using is

def w[T] = writer[Unit].comap[T](_ => ())
def r[T] = reader[Unit].map[T](_ => ???)
def rw[T]: ReadWriter[T] = ReadWriter.join(r, w)
def task = T.input(...)(rw, implicitly[Ctx])

For two reasons

1. This works for any type, and as far as I can tell the actual JSON value is unused.
2. Some of the values I want to use are sensitive (db credentials for the flyway plugin) and I don't want them written to disk unnecessarily.

This second point could probably be expanded into a separate feature request except that the documented behavior of an input task already does pretty much what I want.