Implement PackageRank from lib.rs

[gbaz]

A very impressive amount of research and thought here: https://github.com/rust-lang/rfcs/blob/master/text/1824-crates.io-default-ranking.md

It would be useful to add this as an explicit metric to sort by. 🤔

[Bodigrim]

It seems lib.rs essentially orders by number of recent downloads, or am I missing something? It should be straightforward to add alongside with existing total downloads.

[gbaz]

Crates.io orders by recent downloads, which we have (though inaccurate because of cdn and can do). I should have really linked this, which explains lib.rs uses a much more sophisticated algorithm: https://lib.rs/about

When I made this ticket, I was under the misapprehension the crates ticket somehow contained this algo, but I don't think it does (just related research). The ranking itself can be figured out from the code here: https://gitlab.com/crates.rs/crates.rs/-/blob/main/ranking/src/lib_ranking.rs