Support Cargo dependency `package` attribute

[sin-ack]

Ran into this while dealing with #145. Cargo allows renaming crates so that the package name is different from what's exposed to code: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#renaming-dependencies-in-cargotoml

This should be supported because some packages in the wild actually need this, such as tracing-serde when --cfg tracing_unstable is enabled: https://docs.rs/tracing/latest/tracing/field/fn.valuable.html

[dzbarsky]

Thanks for filing, I'll take a look