Allow WDL workflow inputs to depend on tasks in the workflow

[adamnovak]

The WDL 1.0 spec says that you can do this:

workflow foo {
  input {
    Int x = 10
    Int y = my_task.out
  }

  call my_task as t1 { input: int_in = x }
  call my_task as t2 { input: int_in = y }
}

(For a suitable definition of my_task.)

In addition to the implied ability to refer to the first my_task call as my_task here, this also is explicitly described as running one of these tasks to determine the input used by the other task, unless y is passed in, in which case they can run in parallel.

To support this we need to refactor how Toil thinks about evaluating workflow inputs and turn the individual items into jobs that can happen after jobs within the workflow.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1606