refactor: use registry_key for getting crates.io key

`registry_key` should always be `crates-io` when `is_crates_io()` is true

This also removes the test verifying `Debug` output,
whose format doesn't need to be guaranteed.

src/cargo/core/package_id.rs

@@ -251,43 +251,6 @@ mod tests {
         assert!(PackageId::new("foo", "", repo).is_err());
     }
 
-    #[test]
-    fn debug() {
-        let loc = CRATES_IO_INDEX.into_url().unwrap();
-        let pkg_id = PackageId::new("foo", "1.0.0", SourceId::for_registry(&loc).unwrap()).unwrap();
-        assert_eq!(
-            r#"PackageId { name: "foo", version: "1.0.0", source: "registry `crates-io`" }"#,
-            format!("{:?}", pkg_id)
-        );
-
-        let expected = r#"
-PackageId {
-    name: "foo",
-    version: "1.0.0",
-    source: "registry `crates-io`",
-}
-"#
-        .trim();
-
-        // Can be removed once trailing commas in Debug have reached the stable
-        // channel.
-        let expected_without_trailing_comma = r#"
-PackageId {
-    name: "foo",
-    version: "1.0.0",
-    source: "registry `crates-io`"
-}
-"#
-        .trim();
-
-        let actual = format!("{:#?}", pkg_id);
-        if actual.ends_with(",\n}") {
-            assert_eq!(actual, expected);
-        } else {
-            assert_eq!(actual, expected_without_trailing_comma);
-        }
-    }
-
     #[test]
     fn display() {
         let loc = CRATES_IO_INDEX.into_url().unwrap();

src/cargo/core/source_id.rs

@@ -331,9 +331,7 @@ impl SourceId {
 
     /// Displays the name of a registry if it has one. Otherwise just the URL.
     pub fn display_registry_name(self) -> String {
-        if self.is_crates_io() {
-            CRATES_IO_REGISTRY.to_string()
-        } else if let Some(key) = self.inner.registry_key.as_ref().map(|k| k.key()) {
+        if let Some(key) = self.inner.registry_key.as_ref().map(|k| k.key()) {
             key.into()
         } else if self.precise().is_some() {
             // We remove `precise` here to retrieve an permissive version of