Clean up RunScript helpers and Evaluator code

[lihaoyi]

Not sure exactly how, but they definitely feel a bit out of place, as a bunch of static helpers wrapping Evaluator

Noticed when working on #4091

Would need to be done in 0.13.0 when we can break bincompat

[lihaoyi]

#4144 helps a bit, but more to be done. In particular we can get rid of the Terminal.Task case class and branches since outside of test suites nobody should be running anonymous tasks directly

[lefou]

In particular we can get rid of the Terminal.Task case class and branches since outside of test suites nobody should be running anonymous tasks directly

What about evaluator commands? We heavily use anonymous tasks in GenIdeaImpl for example.

[lihaoyi]

Good question, we'll need to see if there's any alternatives that can be provided, if not we're stuck supporting them whether we like it or not

[lihaoyi]

I think what needs to happen here is to split up the internal Evaluator component from the user-facing Evaluator component. The naming is a bit problematic with internal and external Evaluators, we probably need a new name for one of them. Maybe Executor or something?

1. The internal Evaluator takes a Seq[Task_]] graph, evaluates it, and returns the resultant values
2. The user-facing Executor should encapsulate all of the user facing APIs: Resolve, Plan, Evaluator, etc. some of which is currently in random RunScript helpers

The goal being that user-land tasks should not need to dig around the various packages in mill.{eval,resolve,main} to do stuff, and should be presented a single Executor object that exposes the APIs they need

[lihaoyi]

Made a lot of progress here, but more to be done. All user-facing APIs have been consolidated into Evaluator, though the signatures themselves are still very messy e.g.

  def resolveEvaluate(
      scriptArgs: Seq[String],
      selectMode: SelectMode,
      selectiveExecution: Boolean = false
  ): Result[(Seq[Watchable], Result[Seq[(Any, Option[(Evaluator.TaskName, ujson.Value)])]])]

  def evaluate(
      targets: Seq[Task[Any]],
      selectiveExecution: Boolean = false
  ): (Seq[Watchable], Result[Seq[(Val, Option[(Evaluator.TaskName, ujson.Value)])]]) = {