Replace internal TarIndex with `tar` package

I think that the TarIndex stuff in the `/Data` directory (and in fact the whole `Data` directory) now all exists directly in the `tar` package, and we could rip it out entirely from Hackage? Maybe in two steps? First convert existing stuff into a re-export wrapper, and then once we're confident, just find-and-replace to the real deal.

Start by locating TarIndex and the other code under the /Data directory, then compare their interfaces with the tar package. The issue proposes first creating a re-export wrapper and later replacing usages with the package directly; done means the internal implementation is removed after those usages are converted.

Written by the indexing model from the issue text.