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]]
