Use install_only python archive (#4843)

## Summary

Resolves #4834

## Test Plan

```sh
# 3.12.3 is a `install_only` archive
$ cargo run -- python install --preview --force 3.12.3

# 3.9.4 has only `full` archive
$ cargo run -- python install --preview --force 3.9.4
```

crates/uv-python/fetch-download-metadata.py

@@ -33,6 +33,7 @@ HEADERS = {
 }
 VERSIONS_FILE = SELF_DIR / "download-metadata.json"
 FLAVOR_PREFERENCES = [
+    "install_only",
     "shared-pgo",
     "shared-noopt",
     "shared-noopt",
@@ -44,7 +45,6 @@ FLAVOR_PREFERENCES = [
 HIDDEN_FLAVORS = [
     "debug",
     "noopt",
-    "install_only",
 ]
 SPECIAL_TRIPLES = {
     "macos": "x86_64-apple-darwin",

crates/uv-python/src/downloads.rs

@@ -463,12 +463,17 @@ impl ManagedPythonDownload {
         }
 
         // Extract the top-level directory.
-        let extracted = match uv_extract::strip_component(temp_dir.path()) {
+        let mut extracted = match uv_extract::strip_component(temp_dir.path()) {
             Ok(top_level) => top_level,
             Err(uv_extract::Error::NonSingularArchive(_)) => temp_dir.into_path(),
             Err(err) => return Err(Error::ExtractError(filename.to_string(), err)),
         };
 
+        // If the distribution is a `full` archive, the Python installation is in the `install` directory.
+        if extracted.join("install").is_dir() {
+            extracted = extracted.join("install");
+        }
+
         // Persist it to the target
         debug!("Moving {} to {}", extracted.display(), path.user_display());
         rename_with_retry(extracted, &path)

crates/uv-python/src/managed.rs

@@ -252,14 +252,23 @@ impl ManagedPythonInstallation {
     /// The path to this toolchain's Python executable.
     pub fn executable(&self) -> PathBuf {
         if cfg!(windows) {
-            self.path.join("install").join("python.exe")
+            self.python_dir().join("python.exe")
         } else if cfg!(unix) {
-            self.path.join("install").join("bin").join("python3")
+            self.python_dir().join("bin").join("python3")
         } else {
             unimplemented!("Only Windows and Unix systems are supported.")
         }
     }
 
+    fn python_dir(&self) -> PathBuf {
+        let install = self.path.join("install");
+        if install.is_dir() {
+            install
+        } else {
+            self.path.clone()
+        }
+    }
+
     /// The [`PythonVersion`] of the toolchain.
     pub fn version(&self) -> PythonVersion {
         self.key.version()
@@ -307,10 +316,9 @@ impl ManagedPythonInstallation {
     pub fn ensure_externally_managed(&self) -> Result<(), Error> {
         // Construct the path to the `stdlib` directory.
         let stdlib = if cfg!(windows) {
-            self.path.join("install").join("Lib")
+            self.python_dir().join("Lib")
         } else {
-            self.path
-                .join("install")
+            self.python_dir()
                 .join("lib")
                 .join(format!("python{}", self.key.version().python_version()))
         };