These commits move unit related fields of
HscEnv (EPS/HPT/etc.) into
This is preliminary work to support multiple
UnitEnvs active at the same time. This is needed:
import namespaces (e.g. to implement https://github.com/ghc-proposals/ghc-proposals/pull/412)
Several functions will have to be refactored to use
UnitEnv instead of
HscEnv: this is left for future work.