pub fn is_proc_macro(&self) -> bool {
        match self.build_target(&BuildTargetId::Library) {
            Some(build_target) => matches!(build_target.kind(), BuildTargetKind::ProcMacro),
            None => false,
        }
    }
Returns true if this package is a procedural macro.

For more about procedural macros, see [Procedural
Macros](https://doc.rust-lang.org/reference/procedural-macros.html) in the Rust reference.
guppy/src/graph/graph_impl.rs