Allow multiple version of a package in the same dependency tree about esy-issues

[jordwalke]

^ That was just a suggestion. I'm sure there's more elegant solutions like actually tracking buildTimeOnlyDependencies. Alternatively - perform a detection that there are multiple versions of the same package, and then only if the final executable fails to link then provide possible explanation of why. That is not a totally perfect solution either because some libraries provide dynamically linked libraries, and the errors are deferred to link time! So really proper support of relaxing the error message for multiple versions based on buildTimeOnly dependencies is the best solution and I merely suggested a quick hack.