Add serialization and deserialization for --find-links (#3619)

## Summary

Closes https://github.com/astral-sh/uv/issues/3617.

crates/distribution-types/src/index_url.rs

@@ -6,7 +6,6 @@ use std::str::FromStr;
 
 use itertools::Either;
 use once_cell::sync::Lazy;
-use serde::{Deserialize, Serialize};
 use url::Url;
 
 use pep508_rs::{expand_env_vars, split_scheme, strip_host, Scheme, VerbatimUrl};
@@ -110,7 +109,7 @@ impl serde::ser::Serialize for IndexUrl {
     where
         S: serde::ser::Serializer,
     {
-        self.verbatim().serialize(serializer)
+        self.to_string().serialize(serializer)
     }
 }
 
@@ -159,7 +158,7 @@ impl Deref for IndexUrl {
 /// A directory with distributions or a URL to an HTML file with a flat listing of distributions.
 ///
 /// Also known as `--find-links`.
-#[derive(Debug, Clone, Hash, Eq, PartialEq, Serialize, Deserialize)]
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
 pub enum FlatIndexLocation {
     Path(PathBuf),
     Url(Url),
@@ -185,6 +184,25 @@ impl schemars::JsonSchema for FlatIndexLocation {
     }
 }
 
+impl serde::ser::Serialize for FlatIndexLocation {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::ser::Serializer,
+    {
+        self.to_string().serialize(serializer)
+    }
+}
+
+impl<'de> serde::de::Deserialize<'de> for FlatIndexLocation {
+    fn deserialize<D>(deserializer: D) -> Result<FlatIndexLocation, D::Error>
+    where
+        D: serde::de::Deserializer<'de>,
+    {
+        let s = String::deserialize(deserializer)?;
+        FlatIndexLocation::from_str(&s).map_err(serde::de::Error::custom)
+    }
+}
+
 impl FromStr for FlatIndexLocation {
     type Err = url::ParseError;
 

crates/uv/tests/pip_compile.rs

@@ -8653,6 +8653,36 @@ fn resolve_configuration() -> Result<()> {
     "###
     );
 
+    // Write out a `--find-links` entry.
+    // Add an extra index URL entry to the `pyproject.toml` file.
+    pyproject.write_str(indoc::indoc! {r#"
+        [project]
+        name = "example"
+        version = "0.0.0"
+
+        [tool.uv.pip]
+        no-index = true
+        find-links = ["https://download.pytorch.org/whl/torch_stable.html"]
+    "#})?;
+
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str("tqdm")?;
+
+    uv_snapshot!(context.compile()
+        .arg("requirements.in"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z requirements.in
+    tqdm==4.66.2
+        # via -r requirements.in
+
+    ----- stderr -----
+    Resolved 1 package in [TIME]
+    "###
+    );
+
     Ok(())
 }