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 (Trac #15336, Trac #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.