Flip --experimental_sibling_repository_layout.

src/main/java/com/google/devtools/build/lib/packages/semantics/BuildLanguageOptions.java

@@ -330,7 +330,7 @@ public final class BuildLanguageOptions extends OptionsBase {
 
   @Option(
       name = "experimental_sibling_repository_layout",
-      defaultValue = "false",
+      defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.STARLARK_SEMANTICS,
       effectTags = {
         OptionEffectTag.ACTION_COMMAND_LINES,