Move module-info.java to src/main/java9 for multi-release JAR

Co-authored-by: slawekjaranowski <[email protected]>

Author: Copilot

pom.xml

@@ -111,27 +111,32 @@
             <artifactId>maven-compiler-plugin</artifactId>
             <executions>
               <execution>
-                <id>default-compile</id>
-                <configuration>
-                  <!-- compile everything to ensure module-info contains right entries -->
-                  <release>9</release>
-                </configuration>
-              </execution>
-              <execution>
-                <id>base-compile</id>
+                <id>compile-java9</id>
                 <goals>
                   <goal>compile</goal>
                 </goals>
-                <!-- recompile everything for target VM except the module-info.java -->
+                <phase>compile</phase>
                 <configuration>
-                  <release>8</release>
-                  <excludes>
-                    <exclude>module-info.java</exclude>
-                  </excludes>
+                  <release>9</release>
+                  <compileSourceRoots>
+                    <compileSourceRoot>${project.basedir}/src/main/java9</compileSourceRoot>
+                  </compileSourceRoots>
+                  <multiReleaseOutput>true</multiReleaseOutput>
                 </configuration>
               </execution>
             </executions>
           </plugin>
+          <plugin>
+            <groupId>org.apache.maven.plugins</groupId>
+            <artifactId>maven-jar-plugin</artifactId>
+            <configuration>
+              <archive>
+                <manifestEntries>
+                  <Multi-Release>true</Multi-Release>
+                </manifestEntries>
+              </archive>
+            </configuration>
+          </plugin>
         </plugins>
       </build>
     </profile>