util/resolve: add support for PyPI requirements in API client

[cuixq]

This PR adds support for fetching and processing PyPI requirements in the APIClient:

• Implements pypiRequirements method to process PyPI requirements:
  • Extras are mapped to dep.EnabledDependencies attribute.
  • EnvironmentMarker is mapped to dep.Environment attribute.
• Tests are also added to reflect this change.

[tymzd]

Thank you!