Publish: Support --index <name> (#9694)
When publishing, we currently ask the user to set `--publish-url` to the
upload URL and `--check-url` to the simple index URL, or the equivalent
configuration keys. But that's redundant with the `[[tool.uv.index]]`
declaration. Instead, we extend `[[tool.uv.index]]` with a `publish-url`
entry and allow passing `uv publish --index <name>`.
`uv publish --index <name>` requires the `pyproject.toml` to be present
when publishing, unlike using `--publish-url ... --check-url ...` which
can be used e.g. in CI without a checkout step. `--index` also always
uses the check URL feature to aid upload consistency.
The documentation tries to explain both approaches together, which
overlap for the check URL feature.
Fixes #8864
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Co-authored-by: Zanie Blue <contact@zanie.dev>
