Avoid treating non-existent `--find-links` as relative URLs (#9720)

## Summary

Closes https://github.com/astral-sh/uv/issues/9681.

crates/uv-client/src/flat_index.rs

@@ -243,8 +243,12 @@ impl<'a> FlatIndexClient<'a> {
         path: &Path,
         flat_index: &IndexUrl,
     ) -> Result<FlatIndexEntries, FindLinksDirectoryError> {
+        // The path context is provided by the caller.
+        #[allow(clippy::disallowed_methods)]
+        let entries = std::fs::read_dir(path)?;
+
         let mut dists = Vec::new();
-        for entry in fs_err::read_dir(path)? {
+        for entry in entries {
             let entry = entry?;
             let metadata = entry.metadata()?;
 

crates/uv-distribution-types/src/index_url.rs

@@ -1,15 +1,15 @@
-use itertools::Either;
-use rustc_hash::{FxHashMap, FxHashSet};
 use std::borrow::Cow;
 use std::fmt::{Display, Formatter};
 use std::ops::Deref;
-use std::path::Path;
 use std::str::FromStr;
 use std::sync::{Arc, LazyLock, RwLock};
+
+use itertools::Either;
+use rustc_hash::{FxHashMap, FxHashSet};
 use thiserror::Error;
 use url::{ParseError, Url};
 
-use uv_pep508::{VerbatimUrl, VerbatimUrlError};
+use uv_pep508::{split_scheme, Scheme, VerbatimUrl, VerbatimUrlError};
 
 use crate::{Index, Verbatim};
 
@@ -114,10 +114,23 @@ impl FromStr for IndexUrl {
     type Err = IndexUrlError;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let url = if Path::new(s).exists() {
-            VerbatimUrl::from_absolute_path(std::path::absolute(s)?)?
-        } else {
-            VerbatimUrl::parse_url(s)?
+        let url = match split_scheme(s) {
+            Some((scheme, ..)) => {
+                match Scheme::parse(scheme) {
+                    Some(_) => {
+                        // Ex) `https://pypi.org/simple`
+                        VerbatimUrl::parse_url(s)?
+                    }
+                    None => {
+                        // Ex) `C:\Users\user\index`
+                        VerbatimUrl::from_absolute_path(std::path::absolute(s)?)?
+                    }
+                }
+            }
+            None => {
+                // Ex) `/Users/user/index`
+                VerbatimUrl::from_absolute_path(std::path::absolute(s)?)?
+            }
         };
         Ok(Self::from(url.with_given(s)))
     }

crates/uv/tests/it/pip_install.rs

@@ -79,6 +79,41 @@ fn missing_pyproject_toml() {
     );
 }
 
+#[test]
+fn missing_find_links() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str("flask")?;
+
+    let error = regex::escape("The system cannot find the path specified. (os error 3)");
+    let filters = context
+        .filters()
+        .into_iter()
+        .chain(std::iter::once((
+            error.as_str(),
+            "No such file or directory (os error 2)",
+        )))
+        .collect::<Vec<_>>();
+
+    uv_snapshot!(filters, context.pip_install()
+        .arg("-r")
+        .arg("requirements.txt")
+        .arg("--find-links")
+        .arg("./missing")
+        .arg("--strict"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Failed to read `--find-links` directory: [TEMP_DIR]/missing
+      Caused by: No such file or directory (os error 2)
+    "###
+    );
+
+    Ok(())
+}
+
 #[test]
 fn invalid_pyproject_toml_syntax() -> Result<()> {
     let context = TestContext::new("3.12");