Zejun Wu authored Dec 17, 2018 and Ben Gamari committed Dec 17, 2018

We used to enqueue another command to change directory in the external
interpreter subprocess, this is not as robust as:

* it can fail with -fno-implict-import-qualified;
* it doesn't work when we `setGHCiMonad` to something other than `IO`.

Neither of them works if `directory` package is hidden though.

Test Plan:
```
$ inplace/bin/ghc-stage2 --interactive # -fexternal-interpreter
GHCi, version 8.7.20181213: http://www.haskell.org/ghc/  :? for help
Prelude> :cd ..
Prelude> System.Directory.getCurrentDirectory
"/data/users/watashi"
Prelude> :!pwd
/data/users/watashi
Prelude>
Leaving GHCi.
```

  ./validate

Reviewers: simonmar, bgamari, RyanGlScott

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5453