Add checkstyle rule prohibiting imports of guava optional/supplier
<property name="illegalPkgs" value="org.apache.log4j, org.apache.logging.log4j, java.util.logging, ch.qos.logback"/>
<message key="import.illegal" value="Use SLF4J instead of a logging framework directly."/>
</module>
+ <module name="IllegalImport">
+ <property name="illegalClasses" value="com.google.common.base.Optional, com.google.common.base.Supplier"/>
+ <message key="import.illegal" value="Use the Java8 version of Guava objects."/>
+ </module>
<module name="IllegalInstantiation"> <!-- Java Coding Guidelines: Never instantiate primitive types -->
<property name="classes" value="java.lang.Boolean"/>
<property name="classes" value="java.lang.Byte"/>
