Allow to blacklist modules from affecting the analysis result.

[ibabushkin]

We could interpret something like #[doc(hidden)] or accept a config file. Especially serde's private modules would benefit from this.

[eddyb]

It's still risky, so I'd turn errors into warnings for #[doc(hidden)] methods, at most, and wait for macro hygiene to fix this correctly.

[ibabushkin]

Well, we don't emit anything more severe than a warning at the moment - but that can be changed of course. I guess a mechanism akin to the facilities to silence clippy warnings would be an appropriate solution: we'd output the changes recorded, but explicitly show we essentially ignore them.