refactor: --registry always conflicts with --index

Move the validation upfront to clap

src/cargo/util/command_prelude.rs

@@ -291,7 +291,12 @@ pub trait CommandExt: Sized {
     }
 
     fn arg_index(self, help: &'static str) -> Self {
-        self._arg(opt("index", help).value_name("INDEX"))
+        // Always conflicts with `--registry`.
+        self._arg(
+            opt("index", help)
+                .value_name("INDEX")
+                .conflicts_with("registry"),
+        )
     }
 
     fn arg_dry_run(self, dry_run: &'static str) -> Self {

tests/testsuite/alt_registry.rs

@@ -1389,10 +1389,9 @@ fn both_index_and_registry() {
         p.cargo(cmd)
             .arg("--registry=foo")
             .arg("--index=foo")
-            .with_status(101)
-            .with_stderr(
-                "[ERROR] both `--index` and `--registry` \
-                should not be set at the same time",
+            .with_status(1)
+            .with_stderr_contains(
+                "error: the argument '--registry <REGISTRY>' cannot be used with '--index <INDEX>'",
             )
             .run();
     }