Make direct dependency detection respect markers

[charliermarsh]

Summary

When determining "direct" dependencies, we need to ensure that we respect markers. In the linked issue, the user had an optional dependency like:

[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 #2203.

Test Plan

cargo test