Add dependents ("via ..." comments) in export command (#12350)

Adding dependency trace/parent comments ("via ...") to the export
command output.
This is a similar behavior to the pip compile output.

#### Note to the eager reviewer:
First of all - thanks!  
Secondly, this is still a very rough draft. These are the first lines of
code I've ever written in Rust. This is still mostly an educational/fun
exercise for myself. If opening a Draft PR is creating too much noise -
I apologize and I will close it until it is ready.

## Summary

Resolves #7777

## Test Plan

- [X] manual command execution
- [x] update expected output in tests

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/uv-resolver/src/lock/requirements_txt.rs

@@ -5,6 +5,7 @@ use std::fmt::Formatter;
 use std::path::{Component, Path, PathBuf};
 
 use either::Either;
+use owo_colors::OwoColorize;
 use petgraph::graph::NodeIndex;
 use petgraph::prelude::EdgeRef;
 use petgraph::visit::IntoNodeReferences;
@@ -41,6 +42,7 @@ impl<'lock> RequirementsTxtExport<'lock> {
         prune: &[PackageName],
         extras: &ExtrasSpecification,
         dev: &DependencyGroupsWithDefaults,
+        annotate: bool,
         editable: EditableMode,
         hashes: bool,
         install_options: &'lock InstallOptions,
@@ -312,6 +314,21 @@ impl<'lock> RequirementsTxtExport<'lock> {
             .map(|(index, package)| Requirement {
                 package,
                 marker: reachability.remove(&index).unwrap_or_default(),
+                dependents: if annotate {
+                    let mut dependents = graph
+                        .edges_directed(index, Direction::Incoming)
+                        .map(|edge| &graph[edge.source()])
+                        .filter_map(|node| match node {
+                            Node::Package(package) => Some(*package),
+                            Node::Root => None,
+                        })
+                        .collect::<Vec<_>>();
+                    dependents.sort_unstable_by_key(|package| package.name());
+                    dependents.dedup_by_key(|package| package.name());
+                    dependents
+                } else {
+                    Vec::new()
+                },
             })
             .filter(|requirement| !requirement.marker.is_false())
             .collect::<Vec<_>>();
@@ -496,7 +513,12 @@ fn conflict_marker_reachability<'lock>(
 impl std::fmt::Display for RequirementsTxtExport<'_> {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         // Write out each package.
-        for Requirement { package, marker } in &self.nodes {
+        for Requirement {
+            package,
+            marker,
+            dependents,
+        } in &self.nodes
+        {
             match &package.id.source {
                 Source::Registry(_) => {
                     let version = package
@@ -586,6 +608,20 @@ impl std::fmt::Display for RequirementsTxtExport<'_> {
             }
 
             writeln!(f)?;
+
+            // Add "via ..." comments for all dependents.
+            match dependents.as_slice() {
+                [] => {}
+                [dependent] => {
+                    writeln!(f, "{}", format!("    # via {}", dependent.id.name).green())?;
+                }
+                _ => {
+                    writeln!(f, "{}", "    # via".green())?;
+                    for &dependent in dependents {
+                        writeln!(f, "{}", format!("    #   {}", dependent.id.name).green())?;
+                    }
+                }
+            }
         }
 
         Ok(())
@@ -637,6 +673,7 @@ impl Reachable<MarkerTree> for Edge<'_> {
 struct Requirement<'lock> {
     package: &'lock Package,
     marker: MarkerTree,
+    dependents: Vec<&'lock Package>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord)]