`Krates::direct_dependencies` returns transitive dependencies enabled by feature-gates

**Describe the bug**
When a feature-gate enables a new transitive dependency via a direct dependency, the transitive dependency is listed in the output of `Krates::direct_dependencies`.

**To Reproduce**
A minimal reproduction exists in this repo: https://github.com/BatmanAoD/krates-direct-deps-bug

To reproduce from scratch:

* create a set of crates such that:
* `a` depends on `b`
* `b` has a feature gate that enables one or more dependencies that `a` does not depend on directly
* use `Krates::direct_dependencies` to get the direct dependencies of `a`

**Expected behavior**
Only dependencies listed in the manifest for `a` should be included.

**Device:**
- OS: MacOS
- Version: v0.21.3

Start by running the minimal reproduction linked in the issue and inspect the Krates::direct_dependencies entry point. Trace how feature-gated transitive dependencies enter the result when one crate depends on another. Done means the output contains only dependencies listed directly in the requesting crate's manifest, including for the described feature-gate setup.

Written by the indexing model from the issue text.