Initialize all --prefix subdirectories (#4895)

## Summary

We need to initialize the same directories that we create in `bare.rs`,
since the installer expects them to exist.

Closes #4892.

crates/uv-python/src/environment.rs

@@ -127,23 +127,21 @@ impl PythonEnvironment {
     }
 
     /// Create a [`PythonEnvironment`] from an existing [`Interpreter`] and `--target` directory.
-    #[must_use]
-    pub fn with_target(self, target: Target) -> Self {
+    pub fn with_target(self, target: Target) -> std::io::Result<Self> {
         let inner = Arc::unwrap_or_clone(self.0);
-        Self(Arc::new(PythonEnvironmentShared {
-            interpreter: inner.interpreter.with_target(target),
+        Ok(Self(Arc::new(PythonEnvironmentShared {
+            interpreter: inner.interpreter.with_target(target)?,
             ..inner
-        }))
+        })))
     }
 
     /// Create a [`PythonEnvironment`] from an existing [`Interpreter`] and `--prefix` directory.
-    #[must_use]
-    pub fn with_prefix(self, prefix: Prefix) -> Self {
+    pub fn with_prefix(self, prefix: Prefix) -> std::io::Result<Self> {
         let inner = Arc::unwrap_or_clone(self.0);
-        Self(Arc::new(PythonEnvironmentShared {
-            interpreter: inner.interpreter.with_prefix(prefix),
+        Ok(Self(Arc::new(PythonEnvironmentShared {
+            interpreter: inner.interpreter.with_prefix(prefix)?,
             ..inner
-        }))
+        })))
     }
 
     /// Returns the root (i.e., `prefix`) of the Python interpreter.

crates/uv-python/src/interpreter.rs

@@ -124,21 +124,21 @@ impl Interpreter {
     }
 
     /// Return a new [`Interpreter`] to install into the given `--target` directory.
-    #[must_use]
-    pub fn with_target(self, target: Target) -> Self {
-        Self {
+    pub fn with_target(self, target: Target) -> io::Result<Self> {
+        target.init()?;
+        Ok(Self {
             target: Some(target),
             ..self
-        }
+        })
     }
 
     /// Return a new [`Interpreter`] to install into the given `--prefix` directory.
-    #[must_use]
-    pub fn with_prefix(self, prefix: Prefix) -> Self {
-        Self {
+    pub fn with_prefix(self, prefix: Prefix) -> io::Result<Self> {
+        prefix.init(self.virtualenv())?;
+        Ok(Self {
             prefix: Some(prefix),
             ..self
-        }
+        })
     }
 
     /// Return the [`Interpreter`] for the base executable, if it's available.

crates/uv-python/src/prefix.rs

@@ -25,8 +25,10 @@ impl Prefix {
     }
 
     /// Initialize the `--prefix` directory.
-    pub fn init(&self) -> std::io::Result<()> {
-        fs_err::create_dir_all(&self.0)?;
+    pub fn init(&self, virtualenv: &Scheme) -> std::io::Result<()> {
+        for site_packages in self.site_packages(virtualenv) {
+            fs_err::create_dir_all(site_packages)?;
+        }
         Ok(())
     }