Search in both purelib and platlib for site-packages population (#2537)

## Summary

In reality, there's no such thing as the `site-packages` directory for a
given virtualenv. Rather, Python defines both `purelib` and `platlib`,
where the former is for pure-Python packages and the latter is for
packages that contain native code. These are almost always set to the
same thing... but they don't _have_ to be, and in fact of Fedora they
are not.

This PR changes the `site_packages` method to return an iterator of
directories.

Closes https://github.com/astral-sh/uv/issues/2527.

crates/uv-interpreter/src/python_environment.rs

@@ -88,9 +88,18 @@ impl PythonEnvironment {
         self.interpreter.sys_executable()
     }
 
-    /// Returns the path to the `site-packages` directory inside a virtual environment.
-    pub fn site_packages(&self) -> &Path {
-        self.interpreter.purelib()
+    /// Returns an iterator over the `site-packages` directories inside a virtual environment.
+    ///
+    /// In most cases, `purelib` and `platlib` will be the same, and so the iterator will contain
+    /// a single element; however, in some distributions, they may be different.
+    pub fn site_packages(&self) -> impl Iterator<Item = &Path> {
+        std::iter::once(self.interpreter.purelib()).chain(
+            if self.interpreter.purelib() == self.interpreter.platlib() {
+                None
+            } else {
+                Some(self.interpreter.platlib())
+            },
+        )
     }
 
     /// Returns the path to the `bin` directory inside a virtual environment.