Retain ic_monad and ic_int_print in InteractiveContext after load
when they are defined in external packages. This is supposed to be
the desired behavior that the interactive-print and setGHCiMonad
will survive after :cd, :add, :load, :reload and :set in GHCi.

Test Plan:
Install a interactive-print function and GHCi monad from extenal
pacakge. Try :cd, :load and other commands, make sure that the
interactive-print function and GHCi monad always keep the same.

Reviewed By: simonmar

Differential Revision: https://phabricator.haskell.org/D867