Avoid loading needless proc-macro dependencies

Fixes #37958 when no proc-macros are exported; in particular, without `pub extern crate proc_macros;`, `#![feature(macro_reexport)]`, or `#![feature(use_extern_macros)]`.

I opened https://github.com/rust-lang/cargo/issues/3334 for exported proc macros.

r? @alexcrichton