Impl item removals should lint if the fallback item is not public API.

[obi1kenobi]

Before this PR, the removal of the marked method in this snippet did not cause an inherent_method_missing lint:

pub struct SecretlyIterator;

impl SecretlyIterator {
    // This method gets removed.
    pub fn next(&mut self) -> Option<<Self as Iterator>::Item> {
        <Self as Iterator>::next()
    }
}

#[doc(hidden)]
impl Iterator for SecretlyIterator {
    type Item = i64;
    
    fn next(&mut self) -> Option<Self::Item> {
        None
    }
}

The reasoning was that the impl Iterator has a matching method and Iterator is in scope by default (assuming the prelude is used), so users of this code wouldn't get a compile error. Instead, they'd automatically fall back to the Iterator::next() method through the magic of Rust method resolution.

With this PR, we improve the analysis for methods/associated functions and associated constants, so that cases like this do cause those lints — it'll now ignore trait implementations marked #[doc(hidden)] when considering if there's a valid fallback for the item that was removed.