in-place post-link steps

We're allowing in place post-link steps.
In place postlink steps:

• special kind of Nodes (reference to nodes which point to the same filename)
  example:
  {signed-executable} <- (executable) <- (object) <- (source)
  
  The build graph executor has to take care that all transformers that depend on (executable) have
  been run before it creates {signed-executable}

  .
  There can only a line of

  {a}

  <-

  {b}

  <-(c) there cannot be another

  {d}

  <-(c). When creating the buildgraph we must check that condition.