Refactor runtime interpreter code

In #14335 we want to be able to use both the internal interpreter (for
the plugins) and the external interpreter (for TH and GHCi) at the same
time.

This patch performs some preliminary refactoring: the `hsc_interp` field
of HscEnv replaces `hsc_iserv` and is now used to indicate which
interpreter (internal, external) to use to execute TH and GHCi.

Opt_ExternalInterpreter flag and iserv options in DynFlags are now
queried only when we set the session DynFlags. It should help making GHC
multi-target in the future by selecting an interpreter according to the
selected target.