Reapply "Apply flatten plugin."

This reverts commit bf46e87.

Author: ryanjbaxter

pom.xml

@@ -86,6 +86,10 @@
 		            <argLine>--add-opens java.base/java.util=ALL-UNNAMED</argLine>
 		       </configuration>
 		     </plugin>
+			<plugin>
+				<groupId>org.codehaus.mojo</groupId>
+				<artifactId>flatten-maven-plugin</artifactId>
+			</plugin>
 		</plugins>
 		<pluginManagement>
 			<plugins>