* Enforce downward closure rule (first attempt :-)
  If both object and source of a module are available, be simple and
  choose the more recent.  If that causes a subsequent violation of the
  DCR, complain to the user at the time the violation is detected.
  The alternative is to have a clever algorithm which makes clever
  choices now to avoid conflicts later, but that looks complicated to
  do, and I think it would also confuse users.

* As a side effect of the above, enforce the rule that the Prelude
  must be all source or all object, but not a combination.

* Rationalise signature and semantics for findFilesForModule, so as to
  make client code simpler.