Hide implementation details (java-native-access#689)

Motivation:

We should hide the annotation so someone will not import it by mistake

Modifications:

Move annotation to same package as the rest of the code and mark it as
package-private

Result:

Don't expose implementation details

codec-classes-quic/src/main/java/io/netty/incubator/codec/quic/internal/NotNullByDefault.java → codec-classes-quic/src/main/java/io/netty/incubator/codec/quic/NotNullByDefault.java

@@ -13,7 +13,7 @@
  * License for the specific language governing permissions and limitations
  * under the License.
  */
-package io.netty.incubator.codec.quic.internal;
+package io.netty.incubator.codec.quic;
 
 
 import org.jetbrains.annotations.NotNull;
@@ -39,5 +39,5 @@
 @Retention(RetentionPolicy.CLASS)
 @Target(ElementType.PACKAGE)
 @NotNull
-public @interface NotNullByDefault {
+@interface NotNullByDefault {
 }

codec-classes-quic/src/main/java/io/netty/incubator/codec/quic/package-info.java

@@ -19,4 +19,3 @@
 @NotNullByDefault
 package io.netty.incubator.codec.quic;
 
-import io.netty.incubator.codec.quic.internal.NotNullByDefault;