Pin named indexes in `uv add` (#7747)
## Summary
This PR adds an index pin with `uv add` when the user provides exactly
one named index. We don't pin if the user provides an unnamed index, or
if they provide multiple indexes.
We probably _could_ pin on multiple indexes by writing the sources
_after_ resolution, if that's desirable. But we have no idea which index
the user _expects_ each package to come from.
Possible extensions:
- `uv add --no-pin` to avoid this pinning.
- Warn if they provide a single, unnamed index? I'm not sure if that's
worth a warn. Open to input.
