Allow multiple self-dependencies
In uv, we don't use the `DependencyConstraints` map, but pass in the dependencies through an iterator. This means there can be duplicate dependencies in the input. This would previously make `merge_dependents` panic if a package dependent on itself twice with the same range:
```toml
[package]
name = "foo"
version = "0.1.0"
dependencies = ["foo", "foo"]
```
The fix is to ignore self-dependencies when merging dependents, given that they are always trivially true or trivially false.