First reported in rust-lang/rust#65014 it looks like our error message
on cyclic dependencies may be confusing at times. It looks like this is
an issue because there are multiple paths through a graph for a
dependency, so using the generic `path_to_top` function isn't producing
the most useful path for this purpose.
We're already walking the graph though, so this commit adds an extra
parameter which collects the list of packages we've visited so far to
produce a hopefully always-accurate error message showing the chain of
dependencies end-to-end for what depends on what.
These tests take a good amount of time to run locally and they're also
causing a lot of dependencies to get pulled into rust-lang/rust, so
let's have a separate crate that we just test on our own CI
