bazel syntax: rename SkylarkNestedSet to Depset

The new name matches the Bazel documentation, whereas
NestedSet is a Blaze internal. Also, it is shorter.

Also: tweak its class doc comment.
All other changes are mechanical.
PiperOrigin-RevId: 281824680
diff --git a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleClassFunctionsTest.java b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleClassFunctionsTest.java
index ad00cd9..9d650c2 100644
--- a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleClassFunctionsTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleClassFunctionsTest.java
@@ -47,12 +47,12 @@
 import com.google.devtools.build.lib.skyframe.SkylarkImportLookupFunction;
 import com.google.devtools.build.lib.skylark.util.SkylarkTestCase;
 import com.google.devtools.build.lib.syntax.ClassObject;
+import com.google.devtools.build.lib.syntax.Depset;
 import com.google.devtools.build.lib.syntax.Dict;
 import com.google.devtools.build.lib.syntax.EvalException;
 import com.google.devtools.build.lib.syntax.EvalUtils;
 import com.google.devtools.build.lib.syntax.Mutability;
 import com.google.devtools.build.lib.syntax.ParserInput;
-import com.google.devtools.build.lib.syntax.SkylarkNestedSet;
 import com.google.devtools.build.lib.syntax.StarlarkFile;
 import com.google.devtools.build.lib.syntax.StarlarkList;
 import com.google.devtools.build.lib.syntax.SyntaxError;
@@ -1260,7 +1260,7 @@
   @Test
   public void testNsetGoodCompositeItem() throws Exception {
     exec("def func():", "  return depset([struct(a='a')])", "s = func()");
-    Collection<?> result = ((SkylarkNestedSet) lookup("s")).toCollection();
+    Collection<?> result = ((Depset) lookup("s")).toCollection();
     assertThat(result).hasSize(1);
     assertThat(result.iterator().next()).isInstanceOf(StructImpl.class);
   }