uv-resolver: add Clone impls

For $reasons, we'll want to be able to clone a `Manifest` so
that it can be re-used to generate a marker expression.

There is likely a refactoring that could be done to avoid the
cloning, but a `Manifest` is likely to be small in practice, and
we'll only need to clone it once.

crates/uv-resolver/src/manifest.rs

@@ -6,7 +6,7 @@ use uv_normalize::PackageName;
 use crate::preferences::Preference;
 
 /// A manifest of requirements, constraints, and preferences.
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct Manifest {
     pub(crate) requirements: Vec<Requirement>,
     pub(crate) constraints: Vec<Requirement>,

crates/uv-resolver/src/preferences.rs

@@ -19,7 +19,7 @@ pub enum PreferenceError {
 }
 
 /// A pinned requirement, as extracted from a `requirements.txt` file.
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct Preference {
     requirement: Requirement,
     hashes: Vec<Hashes>,