Respect preferences for explicit index dependencies from requirements.txt (#10690)

## Summary

Closes: https://github.com/astral-sh/uv/issues/10383.

crates/uv-resolver/src/candidate_selector.rs

@@ -188,10 +188,9 @@ impl CandidateSelector {
             .filter(|(marker, _index, _version)| !env.included_by_marker(marker.pep508()));
         let preferences = preferences_match.chain(preferences_mismatch).filter_map(
             |(marker, source, version)| {
-                // If the package is mapped to an explicit index, only consider preferences that
-                // match the index.
+                // Ignore preferences that are associated with conflicting indexes.
                 index
-                    .map_or(true, |index| source == Some(index))
+                    .is_none_or(|index| source.is_none_or(|source| source == index))
                     .then_some((marker, version))
             },
         );

crates/uv/tests/it/pip_compile.rs

@@ -14345,3 +14345,51 @@ fn max_python_requirement() -> Result<()> {
 
     Ok(())
 }
+
+/// See: <https://github.com/astral-sh/uv/issues/10383>
+#[test]
+fn respect_index_preference() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc::indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        dependencies = ["iniconfig>=1", "typing-extensions>=4"]
+
+        [[tool.uv.index]]
+        name = "pypi"
+        url = "https://pypi.org/simple"
+        explicit = true
+
+        [tool.uv.sources]
+        iniconfig = { index = "pypi" }
+    "#})?;
+
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str(indoc::indoc! {r"
+        iniconfig==1.1.1
+        typing-extensions==4.6.0
+    "})?;
+
+    uv_snapshot!(context
+        .pip_compile()
+        .arg("pyproject.toml")
+        .arg("-o")
+        .arg("requirements.txt"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] pyproject.toml -o requirements.txt
+    iniconfig==1.1.1
+        # via project (pyproject.toml)
+    typing-extensions==4.6.0
+        # via project (pyproject.toml)
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    "###);
+
+    Ok(())
+}