Auto merge of #13322 - RuairidhWilliamson:anon-trait-import, r=y21

Unused trait imports (formerly anonymous trait import)

For #11969

I'm looking for help and feedback on implementing a new lint for suggesting `use ... as _` for traits where possible.

I have had a go at implementing this but I don't know if this is the best way to do it as I am new to clippy.

There are some edge cases I can think of where this doesn't work but have aired on the side of false negatives instead of false positives.

An example of a false negative. I couldn't figure out the best way to resolve an import from within clippy. The sub module imports MyAny so that cannot be anonymized but `use std::any::Any` could be. In this case it is not caught because `Any` and `MyAny` have the same DefId.
```rust
mod nested_mod_used_bad1 {
    use std::any::Any;
    use std::any::Any as MyAny;
    mod foo {
        use crate::nested_mod_used_bad1::MyAny;
        fn foo() {
            println!("{:?}", MyAny::type_id("foo"));
        }
    }
}
```

Any feedback is much appreciated.

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changelog: new lint: `unused_trait_names`

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