Reject struct field names that collide with #[serde(tag)]

[SebTardif]

Summary

Reject named-struct field (and alias) names that collide with #[serde(tag = "...")], matching the existing check for enum struct variants.

Problem

Enums with an internal tag already error at derive time when a field serialize name or alias equals the tag:

error: variant field name `conflict` conflicts with internal tag

Named structs with the same attribute skipped that check. Serialize injects the type name under the tag key, then also emits the colliding field, which produces duplicate keys and breaks round-trips.

#[derive(Serialize)]
#[serde(tag = "type")]
struct S {
    #[serde(rename = "type")]
    x: i32,
}
// Was: {"type":"S","type":0}
// Deserialize of that JSON fails: invalid type string for i32

This early return for structs has been present since the internal-tag field conflict check was introduced (2018).

Change

• Extend check_internal_tag_field_name_conflict to named structs (Style::Struct)
• Skip flattened fields (they never appear under their own name; e.g. tag = "data" with #[serde(flatten)] data)
• Share the rename/alias comparison helper with the enum path
• UI tests: internal-tag-struct.rs and internal-tag-struct-alias.rs next to the existing enum cases

Validation

+ Red: without the fix, colliding struct derives successfully (bug present)
+ Green: with the fix, derive errors with `field name \`conflict\` conflicts with internal tag`
+ cargo +nightly test --test compiletest ui  (120 UI cases, including new ones)
+ cargo +nightly test --test test_macros test_internally_tagged_struct_with_flattened_field

Flatten + tag with a matching field name still compiles (regression covered by the flatten skip).