starlark: rename ClassObject to Structure

Neither class nor object are terms that have any meaning in Starlark.

Starlark.classType(Structure.class) now reports "structure" not "ClassObject"
in error messages relating to parameters of that type.

PiperOrigin-RevId: 340693968
diff --git a/src/main/java/com/google/devtools/build/lib/packages/Attribute.java b/src/main/java/com/google/devtools/build/lib/packages/Attribute.java
index ae039c1..4f35aeb 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/Attribute.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/Attribute.java
@@ -56,11 +56,11 @@
 import java.util.concurrent.atomic.AtomicReference;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
-import net.starlark.java.eval.ClassObject;
 import net.starlark.java.eval.Dict;
 import net.starlark.java.eval.EvalException;
 import net.starlark.java.eval.Starlark;
 import net.starlark.java.eval.StarlarkValue;
+import net.starlark.java.eval.Structure;
 
 /**
  * Metadata of a rule attribute. Contains the attribute name and type, and an default value to be
@@ -1478,7 +1478,7 @@
 
     private Object invokeCallback(EventHandler eventHandler, Map<String, Object> attrValues)
         throws EvalException, InterruptedException {
-      ClassObject attrs =
+      Structure attrs =
           StructProvider.STRUCT.create(
               attrValues, "No such regular (non computed) attribute '%s'.");
       Object result = callback.call(eventHandler, attrs);