This patch is a simple refactoring that prepares for a later one,
related to Trac #10083.

* Add a field tcg_self_boot :: SelfBootInfo to TcGblEnv,
  where SelfBootInfo is a new data type, describing the
  hi-boot file, if any, for the module being compiled.

* Make tcHiBootIface return SelfBootInfo, a new data type

* Make other functions get SelfBootInfo from the monad.

* Remove tcg_mod_name from TcGblEnv; it was barely used and
  simpler to pass around explicitly.