refactor: apply clippy::non_canonical_partial_ord_impl

[shulaoda]

Summary

• In src/cargo-fmt/main.rs, reworks the manual PartialOrd for Target impl so partial_cmp is defined as Some(self.cmp(other)) rather than duplicating the self.path.cmp(&other.path) body that already lives in Ord::cmp.
• Reorders the impl blocks so Ord precedes PartialOrd, matching the canonical pattern.
• Addresses clippy::non_canonical_partial_ord_impl, run as a single-rule pass: cargo clippy -- -A clippy::all -W clippy::non_canonical_partial_ord_impl.

When a type implements both Ord and PartialOrd by hand, partial_cmp must agree with cmp. Routing partial_cmp through cmp removes the risk of the two impls drifting out of sync if the ordering logic is ever changed.

[ytmimi]

Going to close until we decide on which clippy lints we want to enforce in the codebase.