Filter managed Python distributions by platform before querying when included in request (#13936)

In the case where we have platform information in a Python request, we
should filter managed Python distributions by it prior to querying them.

Closes https://github.com/astral-sh/uv/issues/13935

---------

Co-authored-by: Aria Desires <aria.desires@gmail.com>

crates/uv-python/src/lib.rs

@@ -9,6 +9,7 @@ pub use crate::discovery::{
     PythonPreference, PythonRequest, PythonSource, PythonVariant, VersionRequest,
     find_python_installations,
 };
+pub use crate::downloads::PlatformRequest;
 pub use crate::environment::{InvalidEnvironmentKind, PythonEnvironment};
 pub use crate::implementation::ImplementationName;
 pub use crate::installation::{PythonInstallation, PythonInstallationKey};