Use structured wheel tags everywhere (#10542)

## Summary

This PR extends the thinking in #10525 to platform tags, and then uses
the structured tag enums everywhere, rather than passing around strings.
I think this is a big improvement! It means we're no longer doing ad hoc
tag parsing all over the place.

crates/uv-cache/src/lib.rs

@@ -787,7 +787,7 @@ impl CacheBucket {
             Self::Interpreter => "interpreter-v4",
             // Note that when bumping this, you'll also need to bump it
             // in crates/uv/tests/cache_clean.rs.
-            Self::Simple => "simple-v14",
+            Self::Simple => "simple-v15",
             // Note that when bumping this, you'll also need to bump it
             // in crates/uv/tests/cache_prune.rs.
             Self::Wheels => "wheels-v3",