Detect dead code better

[rbtcollins]

We should stop using 'pub' and instead use 'crate' within rustup. Rustup has no exported API, but having pub functions results in dead types and functions not being detected. rust-lang/rust#79061 is an example of this technique applied to rustdoc

[0xPoe]

I am working on this.

[0xPoe]

@rustbot claim

[0xPoe]

• After most of the cleanup is done, we should globally introduce #! [deny(unreachable_pub)].