Rollup merge of #68195 - estebank:impl-trait-2000, r=Centril

Rollup merge of #68195 - estebank:impl-trait-2000, r=Centril

Account for common `impl Trait`/`dyn Trait` return type errors

- When all return paths have the same type, suggest `impl Trait`.
- When all return paths implement the expected `trait`, suggest `Box<dyn Trait>` and mention using an `enum`.
- When multiple different types are returned and `impl Trait` is expected, extend the explanation.
- When return type is `impl Trait` and the return paths do not implement `Trait`, point at the returned values.
- Split `src/librustc/traits/error_reporting.rs` into multiple files to keep size under control.

Fix #68110, cc #66523.