Flag unnecessary visiblity attributes on nested functions

What it does

When a function is nested inside another one, the inner one can only be accessed within the scope of the outer one, but not outside. Yet, it is possible to have a visibility attribute applied to the inner. That conceptually makes little sense. It could be the sign of a bug, wrong conception of the author, or just an oversight. Regardless, it should be uncontroversal that it can be removed:

fn outer() {
  pub(crate) fn inner() {
  }

  inner()
}

I think this could even be a rustc warning, but filing this here for discussion first.

Advantage

• Remove unnecessary (and effectively dead) attributes

Drawbacks

• none (that I am aware of)

Example

fn outer() {
  pub(crate) fn inner() {
  }

  inner()
}

Could be written as:

fn outer() {
  fn inner() {
  }

  inner()
}

Start from the nested-function examples in the issue and review how Clippy implements warnings for Rust visibility attributes. Determine the intended warning behavior for nested functions, including the proposed rustc-warning question. Done means unnecessary visibility attributes are reliably identified without affecting valid outer-scope visibility.