Turn off -Zrustdoc-scrape-examples for proc macro crates

[dtolnay]

Crate name

automod

Build failure link

https://docs.rs/crate/automod/1.0.5/builds/703499

Additional details

It would be extremely unusual for a proc macro crate's implementation to call its own macros. Until rust-lang/cargo#11496 is fixed, or possibly even beyond that point, consider not passing -Zrustdoc-scrape-examples when the crate being documented is a proc macro.

[willcrichton]

This issue will be fixed in rust-lang/cargo#11497.

I would prefer to keep this flag on for all crates rather than special-casing it, since the goal is eventually to have it be a default code pathway within Rustdoc, and it should be robust enough to handle these cases.