Fix oops in scala_compile.
Change how tool bootstrapping uses products.

- Use the "data" products instead of the "file" products.
  They are a better fit for this.
- Register bootstraps against a string key, which we can also
  use for reporting. We previously used the value as the key
  (effectively creating a set instead of a map), which got a little
  confusing when debugging.
- Change reporting to hide the details of bootstrapping by
  default. All those [ivy] stanzas in the output were just confusing.