Allow multiple source entries for each package in tool.uv.sources (#…

…7745)

## Summary

This PR enables users to provide multiple source entries in
`tool.uv.sources`, e.g.:

```toml
[tool.uv.sources]
httpx = [
  { git = "https://github.com/encode/httpx", tag = "0.27.2", marker = "sys_platform == 'darwin'" },
  { git = "https://github.com/encode/httpx", tag = "0.24.1", marker = "sys_platform == 'linux'" },
]
```

The implementation is relatively straightforward: when we lower the
requirement, we now return an iterator rather than a single requirement.
In other words, the above is transformed into two requirements:

```txt
httpx @ git+https://github.com/encode/httpx@0.27.2 ; sys_platform == 'darwin'
httpx @ git+https://github.com/encode/httpx@0.24.1 ; sys_platform == 'linux'
```

We verify (at deserialization time) that the markers are
non-overlapping.

Closes #3397.

crates/pep508-rs/src/marker/tree.rs

@@ -5,12 +5,11 @@ use std::ops::{Bound, Deref};
 use std::str::FromStr;
 
 use itertools::Itertools;
+use pep440_rs::{Version, VersionParseError, VersionSpecifier};
 use pubgrub::Range;
 #[cfg(feature = "pyo3")]
 use pyo3::{basic::CompareOp, pyclass, pymethods};
 use serde::{de, Deserialize, Deserializer, Serialize, Serializer};
-
-use pep440_rs::{Version, VersionParseError, VersionSpecifier};
 use uv_normalize::ExtraName;
 
 use crate::cursor::Cursor;
@@ -1667,6 +1666,28 @@ impl Display for MarkerTreeContents {
     }
 }
 
+#[cfg(feature = "schemars")]
+impl schemars::JsonSchema for MarkerTree {
+    fn schema_name() -> String {
+        "MarkerTree".to_string()
+    }
+
+    fn json_schema(_gen: &mut schemars::gen::SchemaGenerator) -> schemars::schema::Schema {
+        schemars::schema::SchemaObject {
+            instance_type: Some(schemars::schema::InstanceType::String.into()),
+            metadata: Some(Box::new(schemars::schema::Metadata {
+                description: Some(
+                    "A PEP 508-compliant marker expression, e.g., `sys_platform == 'Darwin'`"
+                        .to_string(),
+                ),
+                ..schemars::schema::Metadata::default()
+            })),
+            ..schemars::schema::SchemaObject::default()
+        }
+        .into()
+    }
+}
+
 #[cfg(test)]
 mod test {
     use std::ops::Bound;

crates/uv-distribution/Cargo.toml

@@ -34,6 +34,7 @@ uv-warnings = { workspace = true }
 uv-workspace = { workspace = true }
 
 anyhow = { workspace = true }
+either = { workspace = true }
 fs-err = { workspace = true }
 futures = { workspace = true }
 nanoid = { workspace = true }