Fix Gradle parallel build cache contention in Maven -T builds

When Maven runs with -T N, all modules using the packaging profile invoke
gradle-maven-plugin against the same gradleProjectDirectory
(packaging/java or packaging/js), causing them to share and contend on
the same .gradle/ project cache directory simultaneously.

Two fixes:
- Pass --project-cache-dir pointing to each module's own target/.gradle,
  fully isolating parallel Gradle invocations from each other.
- Add maven-clean-plugin filesets to remove packaging/java/.gradle and
  packaging/js/.gradle on mvn clean. Gradle always writes project-level
  metadata to the project directory regardless of --project-cache-dir,
  so these would otherwise accumulate on CI agents with persistent home
  directories.

pom.xml

@@ -532,6 +532,8 @@
                                     <arg>-PpkgInstallFolder=${pkg.installFolder}</arg>
                                     <arg>-PpkgCopyInstallScripts=${pkg.copyInstallScripts}</arg>
                                     <arg>-PpkgLogFolder=${pkg.unixLogFolder}</arg>
+                                    <arg>--project-cache-dir</arg>
+                                    <arg>${project.build.directory}/.gradle</arg>
                                     <arg>--warning-mode</arg>
                                     <arg>all</arg>
                                 </args>
@@ -888,6 +890,21 @@
                 <groupId>com.mycila</groupId>
                 <artifactId>license-maven-plugin</artifactId>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-clean-plugin</artifactId>
+                <inherited>false</inherited>
+                <configuration>
+                    <filesets>
+                        <fileset>
+                            <directory>${main.dir}/packaging/java/.gradle</directory>
+                        </fileset>
+                        <fileset>
+                            <directory>${main.dir}/packaging/js/.gradle</directory>
+                        </fileset>
+                    </filesets>
+                </configuration>
+            </plugin>
         </plugins>
     </build>