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`

crates/uv-client/src/error.rs

@@ -23,6 +23,11 @@ impl Error {
         *self.kind
     }
 
+    /// Get a reference to the [`ErrorKind`] variant of this error.
+    pub fn kind(&self) -> &ErrorKind {
+        &self.kind
+    }
+
     /// Create a new error from a JSON parsing error.
     pub(crate) fn from_json_err(err: serde_json::Error, url: Url) -> Self {
         ErrorKind::BadJson { source: err, url }.into()