[guppy] handle binary-only dependencies

[sunshowers]

Some dependencies don't have lib targets. While they can't be depended on in stable Rust, there's a nightly Rust feature that enables use of binary-only dependencies. In those cases, the resolved_name is an empty string.

Also added mnemos to our corpus.

[sunshowers]

Note that this isn't a complete fix -- currently we don't have enough information to achieve that. I added this info in a subsequent commit d1e9736

                // This code path is only hit with nightly Rust as of 2023-11. It depends on Rust
                // RFC 3028. at https://github.com/rust-lang/cargo/issues/9096.
                //
                // This isn't quite right -- if we have two or more non-lib dependencies, we'll
                // return true for both of them over here. What we need to do instead is use the
                // extern_name and bin_name fields that are present in nightly DepKindInfo, but that
                // aren't in stable yet. For now, this is the best we can do.
                //
                // (If we're going to be relying on heuristics, it is also possible to use the
                // package ID over here, but that's documented to be an opaque string. It also
                // wouldn't be resilient to patch and replace.)