Detecting and using `GHC_PACKAGE_PATH` would allow "stack exec" support. This way propellor would be able to be built with

    stack build

and run with 

    stack exec -- propellor ...

see [[https://github.com/yesodweb/yesod/issues/1018]] and [[https://github.com/yesodweb/yesod/commit/a7cccf2a7c5df8b26da9ea4fdcb6bac5ab3a3b75]]

> I don't think `stack exec propellor` makes sense to use.
> Instead, `stack install propellor` and then put that in PATH.
> I've now made `propellor --init` know when it was built using stack,
> and it will set up propellor to continue to build itself using stack.
> [[done]] --[[Joey]]
