Curious relationship between change detector and Source

Migrating fixme to an issue:

```rust
//FIXME: it's weird that we call a method on this type, passing
//in an argument we also get from this type? In a world where there
//is a general trait for 'create work' on Source we can have
//a method on change_detector that calls this method with the current inputs?
work: change_detector
.ir_source()
.create_global_metric_work(change_detector.current_inputs())?
.into(),
```