Plugins should either have a builder or a factory method

[ppkarwasz]

Log4j plugin should either have a builder annotated with @PluginBuilderFactory or a factory method annotated with @PluginFactory. As it turns out, some of them have both, for example PatternLayout:

logging-log4j2/log4j-core/src/main/java/org/apache/logging/log4j/core/layout/PatternLayout.java

Lines 301 to 303 in f786b51

	@PluginFactory
	@Deprecated
	public static PatternLayout createLayout(

logging-log4j2/log4j-core/src/main/java/org/apache/logging/log4j/core/layout/PatternLayout.java

Lines 624 to 625 in f786b51

	@PluginBuilderFactory
	public static Builder newBuilder() {

This rule should probably be enforced by the annotation processor.