Fix Interface methods default attributes

[guillep]

Interface methods are not properly marked as abstract or public.
Differently from abstract classes, interfaces can have empty methods, working as abstract method declarations, even though they are not explicitly marked as abstract. Moreover, even though java now allows private interface methods, by default they are public. An example as follows:

public interface Collection {

	//Just an abstract method here 
	int implicitAbstractMethod();
	
	//Just an abstract method here, but explicit 
	abstract int explicitAbstractMethod();

	// Method with body! 
	default int methodWithBody() {
		return 0;
	}
}

In this case, the first two methods should be marked as abstract, the third not.
All of them are public

Now, the current version

• looks at the abstract keyword, only identifying the second one!
• treats interfaces like classes for the default visibility, marking them as package