Check unused explicit imports, e.g. using `ExplicitImports.jl`

One of my packages broke because of

```julia
using Base.Iterators: countfrom, repeat
```

As of Julia 1.9, `repeat` is no longer available that way, and I didn't notice that it was in Base all along.
While checking the other imported identifiers, I noticed some other candidates that didn't even need anymore.
It would be awesome if Aqua could detect those.