Allow multiple self-dependencies (#44)

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.

Author: konstin