alternative to BuildCtx.watchValue for dynamic cross modules

[bishabosha]

this watchValue escape hatch for file IO seems unclean, could it instead be derived from a task?

Either:

• ordinary Task.Sources in the meta build that then have some mechanism to make the value accessible in the next level?
• mechanism to allow to reference tasks as a cross factory - perhaps combined with new kind of Task (so in standard build) that is evaluated before other resolutions?
  other thoughts: cross modules i think are not visible at the top most level of resolution, so i guess tasks should be able to evaluate before the associated cross module factory needs the value