Use serde-untagged to improve some untagged enum error messages (#7822)

## Summary

This is related to https://github.com/astral-sh/uv/issues/7817, but
doesn't close it.

crates/pypi-types/Cargo.toml

@@ -27,6 +27,7 @@ mailparse = { workspace = true }
 regex = { workspace = true }
 rkyv = { workspace = true }
 serde = { workspace = true }
+serde-untagged = { workspace = true }
 thiserror = { workspace = true }
 toml = { workspace = true }
 toml_edit = { workspace = true }

crates/pypi-types/src/simple_json.rs

@@ -1,9 +1,8 @@
 use std::str::FromStr;
 
 use jiff::Timestamp;
-use serde::{Deserialize, Deserializer, Serialize};
-
 use pep440_rs::{VersionSpecifiers, VersionSpecifiersParseError};
+use serde::{Deserialize, Deserializer, Serialize};
 
 use crate::lenient_requirement::LenientVersionSpecifiers;
 
@@ -71,13 +70,24 @@ where
     ))
 }
 
-#[derive(Debug, Clone, Deserialize)]
-#[serde(untagged)]
+#[derive(Debug, Clone)]
 pub enum CoreMetadata {
     Bool(bool),
     Hashes(Hashes),
 }
 
+impl<'de> Deserialize<'de> for CoreMetadata {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        serde_untagged::UntaggedEnumVisitor::new()
+            .bool(|bool| Ok(CoreMetadata::Bool(bool)))
+            .map(|map| map.deserialize().map(CoreMetadata::Hashes))
+            .deserialize(deserializer)
+    }
+}
+
 impl CoreMetadata {
     pub fn is_available(&self) -> bool {
         match self {
@@ -87,24 +97,25 @@ impl CoreMetadata {
     }
 }
 
-#[derive(
-    Debug,
-    Clone,
-    PartialEq,
-    Eq,
-    Hash,
-    Deserialize,
-    rkyv::Archive,
-    rkyv::Deserialize,
-    rkyv::Serialize,
-)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash, rkyv::Archive, rkyv::Deserialize, rkyv::Serialize)]
 #[rkyv(derive(Debug))]
-#[serde(untagged)]
 pub enum Yanked {
     Bool(bool),
     Reason(String),
 }
 
+impl<'de> Deserialize<'de> for Yanked {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        serde_untagged::UntaggedEnumVisitor::new()
+            .bool(|bool| Ok(Yanked::Bool(bool)))
+            .string(|string| Ok(Yanked::Reason(string.to_owned())))
+            .deserialize(deserializer)
+    }
+}
+
 impl Yanked {
     pub fn is_yanked(&self) -> bool {
         match self {