Mark NameOrCurrent as an untagged enum

This changes the required syntax when defining validation rules to be as
documented in the examples.

Expected (works with this change):

    - allow: AlterExistingFiles
      packages:
        - automake

Reality (before this change):

    - allow: AlterExistingFiles
      packages:
        - !Name automake

Signed-off-by: J Robert Ray <jrray@imageworks.com>

crates/spk-schema/src/validation.rs

@@ -237,6 +237,7 @@ pub enum FileAlteration {
 
 /// Either a package name or a special reference to the current package
 #[derive(Debug, Clone, Eq, Hash, Ord, PartialEq, PartialOrd, Deserialize, Serialize)]
+#[serde(untagged)]
 pub enum NameOrCurrent {
     #[serde(rename = "Self")]
     Current,