fix(build): make clean reactor test compilation work

.github/workflows/build.yaml

@@ -32,6 +32,8 @@ jobs:
           exit 1
         fi
         echo "jakarta-test tests encoder-jakarta-jsp $tested from this build"
+    - name: Test clean reactor compilation
+      run: mvn -B -ntp clean test-compile
     - name: Run build
       run: mvn -B -ntp install -PtestJakarta
 

core/pom.xml

@@ -72,6 +72,28 @@
 
     <build>
         <plugins>
+            <!-- Make the multi-release JAR available to downstream reactor modules before they compile their JPMS descriptors. -->
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-jar-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <id>reactor-jar</id>
+                        <phase>process-classes</phase>
+                        <goals>
+                            <goal>jar</goal>
+                        </goals>
+                    </execution>
+                    <!-- The main artifact is already created during process-classes. -->
+                    <execution>
+                        <id>default-jar</id>
+                        <phase>none</phase>
+                        <goals>
+                            <goal>jar</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-surefire-plugin</artifactId>