Use the precise namespace for `Reverse`

[ken-matsui]

What it does

I would say the unnecessary_sort_by lint should use the precise namespace for Reverse.

Currently, it shows help like the following:

help: try: `args.sort_by_key(|b| Reverse(b.name()))`

It is not obvious where Reverse is; for instance, if users' code includes Reverse such as struct or impl, then this suggestion misleads them.
Accordingly, I would suggest using the precise namespace for Reverse as std::cmp::Reverse.

help: try: `args.sort_by_key(|b| std::cmp::Reverse(b.name()))`

Lint Name

unnecessary_sort_by

Example

The current help could be:

 error: use Vec::sort_by_key here instead
   --> $DIR/unnecessary_sort_by.rs:94:9
    |
 LL |         args.sort_by(|a, b| b.name().cmp(&a.name()));
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `args.sort_by_key(|b| Reverse(b.name()))`
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `args.sort_by_key(|b| std::cmp::Reverse(b.name()))`

[ken-matsui]

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