Add an extra struct around the package-to-flat index map (#923)
## Summary
`FlatIndex` is now the thing that's keyed on `PackageName`, while
`FlatDistributions` is what used to be called `FlatIndex` (a map from
version to `PrioritizedDistribution`, for a single package). I find this
a bit clearer, since we can also remove the `from_files` that doesn't
return `Self`, which I had trouble following.
