Migrate StringModule methods to SkylarkCallable from SkylarkSignature.

StringModule is a special case in a number of places because it represents effectively methods which should be annotated on String.class. However, String may not be subclassed in Java. StringModule thus serves as a proxy module for String.

RELNOTES: None.
PiperOrigin-RevId: 192508607
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Runtime.java b/src/main/java/com/google/devtools/build/lib/syntax/Runtime.java
index 2c7b0ee..db7849e 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/Runtime.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Runtime.java
@@ -152,7 +152,7 @@
    */
   public static Class<?> getSkylarkNamespace(Class<?> clazz) {
     return String.class.isAssignableFrom(clazz)
-        ? MethodLibrary.StringModule.class
+        ? StringModule.class
         : EvalUtils.getSkylarkType(clazz);
   }