Add a lookup for the most recent snapshot carrying a package

_Edit: Silly me -- in a Reddit thread, Michael suggested that a "PR" about this would be welcome, and I somehow read that as "feature request". As I probably won't manage to provide the patch myself, feel free to close the issue, mark it as low priority, or handle it however you see fit._
### User story

Let's say I have a Hakyll-powered site that I haven't touched in several months. I want to update it to the latest Hakyll version, which according to [the site of that project](https://jaspervdj.be/hakyll/releases.html) is 4.8.3.2. Since Hakyll is a popular library and I have successfully built my site with stack in the past, I assume I can just change the stack resolver to the recent LTS 7.3, which I have been using with some of my other projects. Then, however, `stack build` fails (a strikingly rare occurrence!). After gathering myself, I think of going to the Stackage site, where I discover that [Hakyll is not yet included in the 7.x snapshots](https://www.stackage.org/diff/lts-6.21/lts-7.3). Though it is only a rather small inconvenience, it would be nice not to have to switch to a web browser and look that up in the site manually.
### Feature suggestion

I suggest there should be a way of querying for at least one of the following:
- The latest snapshot (if any) carrying a version or a range of versions (say, 4.8.*) of one (or several) package(s).
- The latest snapshot (if any) carrying any version of one (or several) package(s), and the version of the package(s) in that snapshot.
That information might then be used by other tools -- for instance, stack would be able to provide more informative error messages if a dependency of a project is missing from a recent snapshot you are trying to use.

Possibly related: #118