Add prerelease compatibility check (#8020)

## Summary

Closes #7977. Makes `PythonDownloadRequest` account for the prerelease
part if allowed. Also stores the prerelease in `PythonInstallationKey`
directly as a `Prerelease` rather than a string.

## Test Plan

Correctly picks the relevant prerelease (rather than picking the most
recent one):
```
λ cargo run python install 3.13.0rc2
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/uv python install 3.13.0rc2`
Searching for Python versions matching: Python 3.13rc2
cpython-3.13.0rc2-macos-aarch64-none ------------------------------ 457.81 KiB/14.73 MiB                                                                                                                    ^C

λ cargo run python install 3.13.0rc3                 
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/uv python install 3.13.0rc3`
Searching for Python versions matching: Python 3.13rc3
Found existing installation for Python 3.13rc3: cpython-3.13.0rc3-macos-aarch64-none
```

crates/uv-python/src/downloads.rs

@@ -260,8 +260,17 @@ impl PythonDownloadRequest {
                 return false;
             }
         }
+        // If we don't allow pre-releases, don't match a key with a pre-release tag
+        if !self.allows_prereleases() && key.prerelease.is_some() {
+            return false;
+        }
         if let Some(version) = &self.version {
-            if !version.matches_major_minor_patch(key.major, key.minor, key.patch) {
+            if !version.matches_major_minor_patch_prerelease(
+                key.major,
+                key.minor,
+                key.patch,
+                key.prerelease,
+            ) {
                 return false;
             }
             if version.is_freethreaded() {
@@ -269,10 +278,6 @@ impl PythonDownloadRequest {
                 return false;
             }
         }
-        // If we don't allow pre-releases, don't match a key with a pre-release tag
-        if !self.allows_prereleases() && !key.prerelease.is_empty() {
-            return false;
-        }
         true
     }