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-python/src/platform.rs

@@ -256,6 +256,10 @@ impl From<&uv_platform_tags::Arch> for Arch {
                 family: target_lexicon::Architecture::S390x,
                 variant: None,
             },
+            uv_platform_tags::Arch::Powerpc => Self {
+                family: target_lexicon::Architecture::Powerpc,
+                variant: None,
+            },
             uv_platform_tags::Arch::Powerpc64 => Self {
                 family: target_lexicon::Architecture::Powerpc64,
                 variant: None,