The problem was that because the interface hadn't changed, we were
re-using the old ModIface.  Unfortunately the ModIface contains the
GlobalRdrEnv for the module, and that *had* changed.  The fix is to
put the new GlobalRdrEnv in the ModIface even if the interface has not
otherwise changed.

ModIface is not really the right place for the GlobalRdrEnv, but
neither is ModDetails, so we should think about a better way to do
this.