Support for extra-package-dbs in 'stack ghci'

[vigoo]

An addition to the extra-package-dbs option (#990), to pass the extra package databases in the stack ghci command too.

[borsboom]

LGTM.

@chrisdone, can review this, since you're more familiar with stack ghci?

[mgsloan]

I've taken a look at this, and I think it should be done a different way.

Note that currently nothing about the package databases is passed to GHCi, yet it works as it should. This is because we're essentially running it via stack exec which sets various environment variables. In particular, it sets GHC_PACKAGE_PATH.

So, I think the best way to fix this (and fix other things along with it!) is to include the extra-package-dbs in GHC_PACKAGE_PATH. This will involve adding an extra argument to mkGhcPackagePath.

[vigoo]

I've rewritten this PR based on @mgsloan 's comment. Please review.

[mgsloan]

Looks good to me, thanks!