[fixes #2838] Handle anonymous classes properly

Generated qualified names (e.g. Outer.Inner) now stop at anonymous
classes instead of adding an empty part. All handlers that add static
fields/methods/types now add error messages instead of generating
invalid code.

test/transform/resource/after-ecj/WithByInAnonymousClass.java

@@ -0,0 +1,20 @@
+import lombok.experimental.WithBy;
+public class WithByInAnonymousClass {
+  Object annonymous = new Object() {
+    @WithBy class Inner {
+      private String string;
+      private Inner(String string) {
+        super();
+      }
+      public @java.lang.SuppressWarnings("all") Inner withStringBy(final java.util.function.Function<? super String, ? extends String> transformer) {
+        return new Inner(transformer.apply(this.string));
+      }
+    }
+    x() {
+      super();
+    }
+  };
+  public WithByInAnonymousClass() {
+    super();
+  }
+}