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, #16563). 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.
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