Add uv export --format requirements.txt (#6778)

## Summary

The interface here is intentionally a bit more limited than `uv pip
compile`, because we don't want `requirements.txt` to be a system of
record -- it's just an export format. So, we don't write annotation
comments (i.e., which dependency is requested from which), we don't
allow writing extras, etc. It's just a flat list of requirements, with
their markers and hashes.

Closes #6007.

Closes #6668.

Closes #6670.

crates/uv-resolver/src/graph_ops.rs

@@ -0,0 +1,81 @@
+use pep508_rs::MarkerTree;
+use petgraph::algo::greedy_feedback_arc_set;
+use petgraph::visit::{EdgeRef, Topo};
+use petgraph::{Directed, Direction, Graph};
+
+/// A trait for a graph node that can be annotated with a [`MarkerTree`].
+pub(crate) trait Markers {
+    fn set_markers(&mut self, markers: MarkerTree);
+}
+
+/// Propagate the [`MarkerTree`] qualifiers across the graph.
+///
+/// The graph is directed, so if any edge contains a marker, we need to propagate it to all
+/// downstream nodes.
+pub(crate) fn propagate_markers<T: Markers>(
+    mut graph: Graph<T, MarkerTree, Directed>,
+) -> Graph<T, MarkerTree, Directed> {
+    // Remove any cycles. By absorption, it should be fine to ignore cycles.
+    //
+    // Imagine a graph: `A -> B -> C -> A`. Assume that `A` has weight `1`, `B` has weight `2`,
+    // and `C` has weight `3`. The weights are the marker trees.
+    //
+    // When propagating, we'd return to `A` when we hit the cycle, to create `1 or (1 and 2 and 3)`,
+    // which resolves to `1`.
+    //
+    // TODO(charlie): The above reasoning could be incorrect. Consider using a graph algorithm that
+    // can handle weight propagation with cycles.
+    let edges = {
+        let mut fas = greedy_feedback_arc_set(&graph)
+            .map(|edge| edge.id())
+            .collect::<Vec<_>>();
+        fas.sort_unstable();
+        let mut edges = Vec::with_capacity(fas.len());
+        for edge_id in fas.into_iter().rev() {
+            edges.push(graph.edge_endpoints(edge_id).unwrap());
+            graph.remove_edge(edge_id);
+        }
+        edges
+    };
+
+    let mut topo = Topo::new(&graph);
+    while let Some(index) = topo.next(&graph) {
+        let marker_tree = {
+            // Fold over the edges to combine the marker trees. If any edge is `None`, then
+            // the combined marker tree is `None`.
+            let mut edges = graph.edges_directed(index, Direction::Incoming);
+
+            edges
+                .next()
+                .and_then(|edge| graph.edge_weight(edge.id()).cloned())
+                .and_then(|initial| {
+                    edges.try_fold(initial, |mut acc, edge| {
+                        acc.or(graph.edge_weight(edge.id())?.clone());
+                        Some(acc)
+                    })
+                })
+                .unwrap_or_default()
+        };
+
+        // Propagate the marker tree to all downstream nodes.
+        let mut walker = graph
+            .neighbors_directed(index, Direction::Outgoing)
+            .detach();
+        while let Some((outgoing, _)) = walker.next(&graph) {
+            if let Some(weight) = graph.edge_weight_mut(outgoing) {
+                weight.and(marker_tree.clone());
+            }
+        }
+
+        let node = &mut graph[index];
+        node.set_markers(marker_tree);
+    }
+
+    // Re-add the removed edges. We no longer care about the edge _weights_, but we do want the
+    // edges to be present, to power the `# via` annotations.
+    for (source, target) in edges {
+        graph.add_edge(source, target, MarkerTree::TRUE);
+    }
+
+    graph
+}