Rename PythonRequest::Any -> PythonRequest::Default (#7514)

As we support more complex Python discovery behaviors such as:

- #7431 
- #7335 
- #7300 

`Any` is no longer accurate, we actually are looking for a reasonable
default Python version to use which may exclude the first one we find.
Separately, we need the idea of `Any` to improve behavior when listing
versions (e.g., #7286) where we do actually want to match _any_ Python
version. As a first step, we'll rename `Any` to `Default`. Then, we'll
introduce a new `Any` that actually behaves as we'd expect.

crates/uv-python/src/downloads.rs

@@ -173,7 +173,7 @@ impl PythonDownloadRequest {
                     .with_version(version.clone()),
             ),
             PythonRequest::Key(request) => Some(request.clone()),
-            PythonRequest::Any => Some(Self::default()),
+            PythonRequest::Default => Some(Self::default()),
             // We can't download a managed installation for these request kinds
             PythonRequest::Directory(_)
             | PythonRequest::ExecutableName(_)