Fix `haskell-interactive-switch` opening a 2nd redundant window

Due to `(haskell-session)` creating (and selecting) a new REPL window
if the session didn't exist prior to its invocation,
`haskell-interactive-switch` needs to check whether the current window
is already showing the REPL buffer before calling
`switch-to-buffer-other-window`.

This should fix #171 which is a regression introduced via fb0168b
(for addressing #155).