True multi package support in "stack ghci"

This is just a stub issue to keep track of the goal of having "stack ghci" properly load packages, including switching cwd when loading modules (for TH file loading). I am removing the support for using intero's `:cd-ghc` command with multiple packages, because it doesn't always work, doesn't work on reload, and was generally a regression in behavior for multi-package projects - see https://github.com/commercialhaskell/stack/issues/3309

A proper solution to this requires an implementation in ghci / intero. Some relevant issues:

https://ghc.haskell.org/trac/ghc/ticket/10827
https://github.com/commercialhaskell/intero/issues/41
https://github.com/commercialhaskell/intero/issues/39