feat: support extra-index-url for pip-repositories

[guenp]

Summary

• Adds support for an extra-index-url field in requirements.yaml that gets translated to pip-repositories in the generated environment.yaml
• This allows conda-lock to resolve pip packages from private registries (e.g., Google Artifact Registry) without manual post-processing
• The field accepts a single URL string or a list of URLs; duplicates from multiple requirements files are deduplicated

Motivation

When using private Python package registries alongside conda-lock, there's no way to specify extra pip indexes through unidep. conda-lock supports a pip-repositories field in environment.yaml, but unidep merge doesn't generate it.

This is needed when internal packages are published to a private registry (e.g., Google Artifact Registry) and need to be resolved by conda-lock's PyPI solver.

Example

requirements.yaml:

extra-index-url: https://us-east4-python.pkg.dev/myproject/myrepo/simple/
dependencies:
  - pip: my-private-package

Generated environment.yaml:

pip-repositories:
  - https://us-east4-python.pkg.dev/myproject/myrepo/simple/
dependencies:
  - pip:
    - my-private-package

Test plan

• Verify unidep merge generates pip-repositories in environment.yaml when extra-index-url is set
• Verify single string and list formats both work
• Verify unidep conda-lock passes the field through to tmp.environment.yaml
• Verify conda-lock can resolve packages from the private registry

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📚 Documentation preview 📚: https://unidep--280.org.readthedocs.build/en/280/