Expose the library

I'm writing some code to interact with Hackage's API. I'm particularly interested in the version information output from [`PackageVersions`](https://github.com/haskell/hackage-server/blob/master/src/Distribution/Server/Features/PackageInfoJSON/State.hs#L120-L188). Since Hackage isn't uploaded, I can't re-use this type directly.

I'm going to work around this by copying and pasting the relevant code, but it seems like it'd be nicer for Hackage to have a library for communication.

It's entirely possible that I've completely missed the appropriate solution for this - if there's an approved way to get this information that *doesn't* involve vendoring secret code, do let me know 😅