Feature Request: allow setup-info urls to be relative file paths

[duog]

Currently if the setup-info url is a relative path stack fails complaining it is not an absolute path.

I want to distribute a custom build of ghc with my project, and reference it from the setup-info url field, however I am prevented from doing so. I instead must use a shell script and call

stack setup --ghc-bindist $(pwd)/bindist/xxx