Make custom julia_version work again

[KristofferC]

Movin out DelmitedFiles from Julia made the custom julia_version code path no longer work since it tries to look up the Project file from the local installation. A discussed solution was to let Pkg accept a path to a custom registry that descirbes the dependencies of the stdlibs over the different Julia versions (but we still need to know what stdlib version was installed for each julia version).