Make PrivateModule#binder() non-final to allow binder customization i…

…n subclasses. Fixes #1569 & fixes #1525. (The primary use-case for this is to override binder() to always call skipSources, like many AbstractModule impls.)

PiperOrigin-RevId: 525231256

core/src/com/google/inject/PrivateModule.java

@@ -137,7 +137,7 @@ protected final AnnotatedElementBuilder expose(TypeLiteral<?> type) {
   // everything below is copied from AbstractModule
 
   /** Returns the current binder. */
-  protected final PrivateBinder binder() {
+  protected PrivateBinder binder() {
     checkState(binder != null, "The binder can only be used inside configure()");
     return binder;
   }