ghci: Don't rely on resolution of System.IO to base module

Previously we would hackily evaluate a textual code snippet to compute actions to disable I/O buffering and flush the stdout/stderr handles. This broke in a number of ways (#15336 (closed), #16563 (closed)).

Instead we now ship a module (GHC.GHCi.Helpers) with base containing the needed actions. We can then easily refer to these via Orig names.