Set maven.multiModuleProjectDirectory as system property

The Maven CLI launcher sets maven.multiModuleProjectDirectory as a JVM
system property (-D) pointing to the directory containing .mvn/. JDTLS
computes this value and injects it as a Maven profile property, but
m2e's MavenExecutionContext.populateSystemProperties() reads from
System.getProperties(), not from profile properties.

This means  in POMs remains
unresolved during model building, breaking projects like Apache Dubbo
and Google Guava that use it in resource directory paths.

Set the property via System.setProperty() to mirror the mvn launcher
behavior. This is only done when the property isn't already set
externally.

Fixes #3760

Signed-off-by: Changyong Gong <chagon@microsoft.com>

Author: chagong

org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/preferences/StandardPreferenceManager.java

@@ -143,6 +143,13 @@ public void update(Preferences preferences) {
 					}
 				}
 			}
+			// Set as JVM system property so m2e's MavenExecutionContext.populateSystemProperties()
+			// picks it up via System.getProperties(). Profile properties alone are not reliably
+			// propagated to the Maven model interpolator during m2e's import flow, causing
+			// ${maven.multiModuleProjectDirectory} in POMs to remain unresolved.
+			if (multiModuleProjectDirectory != null && systemMmpd == null) {
+				System.setProperty(MAVEN_MULTI_MODULE_PROJECT_DIRECTORY, multiModuleProjectDirectory);
+			}
 			updateMavenProjects = updateMavenProjects || !Objects.equals(multiModuleProjectDirectory, oldMultiModuleProjectDirectory) || (oldDisableTest != preferences.isMavenDisableTestClasspathFlag());
 			mavenSettings.getProfiles().removeIf(p -> JAVALS_PROFILE.equals(p.getId()));
 			if (preferences.isMavenDisableTestClasspathFlag() || multiModuleProjectDirectory != null) {