Refactor uv python install (#8701)

Pulling out of #8650 for
readability.

Trying to clean this up to simplify extensions in the future. This is
not a strict refactor, there are behavioral changes here.

- Adds some structs for managing state.
- Addresses some likely inconsistent behavior for weird edge-cases. 
- We fill platform information before checking if a request is
satisfied.
- We error earlier if we can't find a download for the request, i.e.,
even if you somehow have it installed.
- Only reports versions as uninstalled if a download actually replaces
them.
- Moves some of the default output to tracing messages.
- Even if an installation was already satisfied, we'll check that it is
setup properly

crates/uv-python/src/downloads.rs

@@ -423,7 +423,7 @@ pub enum DownloadResult {
 }
 
 impl ManagedPythonDownload {
-    /// Return the first [`PythonDownload`] matching a request, if any.
+    /// Return the first [`ManagedPythonDownload`] matching a request, if any.
     pub fn from_request(
         request: &PythonDownloadRequest,
     ) -> Result<&'static ManagedPythonDownload, Error> {
@@ -433,7 +433,7 @@ impl ManagedPythonDownload {
             .ok_or(Error::NoDownloadFound(request.clone()))
     }
 
-    /// Iterate over all [`PythonDownload`]'s.
+    /// Iterate over all [`ManagedPythonDownload`]s.
     pub fn iter_all() -> impl Iterator<Item = &'static ManagedPythonDownload> {
         PYTHON_DOWNLOADS
             .iter()