Make target Python version an optional field (#4000)

## Summary

Instead of checking if the target and installed version are the same, we
model the data such that the target version is only present if it was
specified by the user. This also means that we correctly say "requested
version" even if the two happen to be the same.

crates/uv-resolver/src/python_requirement.rs

@@ -1,5 +1,5 @@
-use pep508_rs::{MarkerEnvironment, StringVersion};
-use uv_interpreter::Interpreter;
+use pep508_rs::StringVersion;
+use uv_interpreter::{Interpreter, PythonVersion};
 
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub struct PythonRequirement {
@@ -8,21 +8,29 @@ pub struct PythonRequirement {
     /// The target version of Python; that is, the version of Python for which we are resolving
     /// dependencies. This is typically the same as the installed version, but may be different
     /// when specifying an alternate Python version for the resolution.
-    target: StringVersion,
+    ///
+    /// If `None`, the target version is the same as the installed version.
+    target: Option<StringVersion>,
 }
 
 impl PythonRequirement {
-    pub fn from_marker_environment(interpreter: &Interpreter, env: &MarkerEnvironment) -> Self {
+    /// Create a [`PythonRequirement`] to resolve against both an [`Interpreter`] and a
+    /// [`PythonVersion`].
+    pub fn from_python_version(interpreter: &Interpreter, python_version: &PythonVersion) -> Self {
         Self {
             installed: interpreter.python_full_version().clone(),
-            target: env.python_full_version().clone(),
+            target: Some(StringVersion {
+                string: python_version.to_string(),
+                version: python_version.python_full_version(),
+            }),
         }
     }
 
+    /// Create a [`PythonRequirement`] to resolve against an [`Interpreter`].
     pub fn from_interpreter(interpreter: &Interpreter) -> Self {
         Self {
             installed: interpreter.python_full_version().clone(),
-            target: interpreter.python_full_version().clone(),
+            target: None,
         }
     }
 
@@ -32,7 +40,7 @@ impl PythonRequirement {
     }
 
     /// Return the target version of Python.
-    pub fn target(&self) -> &StringVersion {
-        &self.target
+    pub fn target(&self) -> Option<&StringVersion> {
+        self.target.as_ref()
     }
 }