rustdoc: hide private traits in strip-private pass

[tomjakubowski]

Fix #16563

[tomjakubowski]

I've verified that this hides traits which:

• Are private at the crate root or private in a module
• Are public in a private module

And does not hide traits which:

• Are public at the crate root or public in a public module
• Are public in a private module and re-exported

[tomjakubowski]

(saving this here https://gist.github.com/tomjakubowski/3f8b9bf33f1d9c5004da)

[alexcrichton]

Sorry for being a little slow on this, thanks @tomjakubowski! r=me with a rebase.

[sfackler]

It'd be nice to update the missing_doc lint with the same logic.

[tomjakubowski]

Rebased but I'm going to update missing_doc later today.