Refactor self-boot info
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.