feat: implement warnings on NeverClones that implement Copy

[donmai-me]

implements #393

[LukeMathWalker]

The implementation looks good!
Can you add a UI test for it?

You can use this one as a reference.
The test should live in this folder.

[donmai-me]

Are ui tests generated by a pavex command? I noticed that the name in a_warning_is_emitted_for_unused_constructors's cargo.toml is

[package]
name = "app_30248169"

Sorry if I missed a piece of documentation somewhere

[LukeMathWalker]

Not yet, although I should create a helper CLI to bootstrap them!
That's the hash of the crate. The way I usually go about it—name it "app_" and when you run cargo test it'll complain about missing a specific app_XXXXX, then I rename it to that 😅 Not the most polished, I know.