Use Checkstyle to validate import order

Author: garydgregory

src/conf/checkstyle/checkstyle.xml

@@ -63,6 +63,12 @@ limitations under the License.
     <module name="OperatorWrap">
       <property name="option" value="eol" />
     </module>
+    <module name="ImportOrder">
+      <property name="option" value="top"/>
+      <property name="groups" value="java,javax,org"/>
+      <property name="ordered" value="true"/>
+      <property name="separated" value="true"/>
+    </module>
   </module>
 
 </module>