Make PrivateModule#binder() non-final to allow binder customization in subclasses

[vlsi]

One of the cases for overriding #binder() would be calling Binder#skipSources(...)

fixes #1525

[vlsi]

@ronshapiro , sorry for pinging you, however, would you please review and merge the change?

It is kind of sad to wait one year for 1 line change 😭

[vlsi]

@cpovirk , could you please review and merge the fix?

In a nutshell, AbstractModule.binder() method is protected and non-final:

guice/core/src/com/google/inject/AbstractModule.java

Line 74 in c7a74a1

protected Binder binder() {

However, PrivateModule.binder() is protected and final at the same time:

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

Lines 137 to 140 in c7a74a1

	// everything below is copied from AbstractModule
	/** Returns the current binder. */
	protected final PrivateBinder binder() {

I believe that was just a typo, and there's no practical meaning for making PrivateModule.binder() method final.

I listed more details in the linked #1525 (comment) issue.