Detect unused deny `wrappers` use cases

[repi]

We had an exemption of a crate deny rule for another crate that still needed it, which we used the wrappers field for (a bit hacky), like this:

deny = [
    { name = "inventory", wrappers = [ "tract-pulse", "tract-pulse-opl" ] }, # disallow these crates that rely on static initialization order which we've had issues with. tract needs it though.
]

but a later version of those tract-* crates had then finally removed the dependency on inventory and it was no longer reference in our Cargo.lock but we didn't know. Would be nice if cargo-deny would have logged out a warning about an unused wrappers field here for the crates so one can get a heads up and remove it.