AVRO-4023: Move IdlUtils into org.apache.avro.idl (#3039)

Noticed this when building the distributions.

I think it makes sense to move it just into the `idl` namspace.

This package hasn't been released before, so we don't break any
existing APIs.

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.8.0:jar (module-javadocs) on project avro-idl: MavenReportException: Error while generating Javadoc:
[ERROR] Exit code: 2
[ERROR] error: No source files for package org.apache.avro.util
[ERROR] 1 error
[ERROR] Command line was: /usr/lib/jvm/java-21-openjdk-arm64/bin/javadoc -J-Duser.language= -J-Duser.country= -Xdoclint:none @options @packages
[ERROR]
[ERROR] Refer to the generated Javadoc files in '/home/fokko.driesprong/avro/lang/java/idl/target/apidocs' dir.
[ERROR]
[ERROR] -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
[ERROR]
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <args> -rf :avro-idl
```

lang/java/idl/src/main/java/org/apache/avro/idl/IdlSchemaFormatter.java

@@ -21,7 +21,6 @@
 
 import org.apache.avro.Schema;
 import org.apache.avro.SchemaFormatter;
-import org.apache.avro.util.IdlUtils;
 
 public class IdlSchemaFormatter implements SchemaFormatter {
   @Override

lang/java/idl/src/main/java/org/apache/avro/util/IdlUtils.java → lang/java/idl/src/main/java/org/apache/avro/idl/IdlUtils.java

@@ -15,7 +15,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package org.apache.avro.util;
+package org.apache.avro.idl;
 
 import com.fasterxml.jackson.core.JsonGenerator;
 import com.fasterxml.jackson.core.util.MinimalPrettyPrinter;
@@ -57,6 +57,9 @@
 import static java.util.Collections.unmodifiableSet;
 import static java.util.Objects.requireNonNull;
 
+/**
+ * Utils for IDL
+ */
 public final class IdlUtils {
   static final ObjectMapper MAPPER;
   private static final Function<Schema.Field, JsonNode> DEFAULT_VALUE;

lang/java/idl/src/test/java/org/apache/avro/util/IdlUtilsTest.java → lang/java/idl/src/test/java/org/apache/avro/idl/IdlUtilsTest.java

@@ -15,7 +15,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package org.apache.avro.util;
+package org.apache.avro.idl;
 
 import java.io.IOException;
 import java.io.InputStream;
@@ -31,8 +31,6 @@
 import org.apache.avro.JsonProperties;
 import org.apache.avro.Protocol;
 import org.apache.avro.Schema;
-import org.apache.avro.idl.IdlFile;
-import org.apache.avro.idl.IdlReader;
 import org.junit.jupiter.api.Test;
 
 import static java.util.Collections.emptyList;