Make direct dependency detection respect markers (#2207)
## Summary
When determining "direct" dependencies, we need to ensure that we
respect markers. In the linked issue, the user had an optional
dependency like:
```toml
[project.optional-dependencies]
dev = [
"setuptools>=64",
"setuptools_scm>=8"
]
```
By not respecting markers, we tried to resolve `setuptools` to the
lowest-available version. However, since `setuptools>=64` _isn't_
enabled (since it's optional), we won't respect _that_ constraint.
To be consistent, we need to omit optional dependencies just as we will
at resolution time.
Closes https://github.com/astral-sh/uv/issues/2203.
## Test Plan
`cargo test`
