Respect `--no-index` with `--find-links` in `pip sync` (#2692)
## Summary
In `pip sync`, we weren't properly handling cases in which a package
_only_ existed in `--find-links` (e.g., the user passed `--offline` or
`--no-index`).
I plan to explore removing `Finder` entirely to avoid these mismatch
bugs between `pip sync` and other commands, but this is fine for now.
Closes https://github.com/astral-sh/uv/issues/2688.
## Test Plan
`cargo test`
