Additional source directories are not put on classpath

Additional source directories that are included using the build-helper-maven-plugin in a Maven project seem to not be made available to the language server, and are thus not picked up by for instance Go To Declaration and Find References.

Consider the attached example project, consisting of three leaf projects: Child1, Child2 and Child3, organized in two parent projects: ParentA and ParentB, where Child3 has a dependency on Child2, and Child2 has a dependency on Child1.

In the first iteration, stored in before/, Child1 and Child2 is organized under ParentA, and Child 3 is organized under ParentB. In the second iteration, stored in after/, Child2 is merged with Child3, but to avoid changes in the project history, the files are still kept under ParentA's directory and included in Child3 using build-helper-maven-plugin.

Environment

• Operating System: Ubuntu Focal (20.04)
• JDK version: OpenJDK 17.0.10
• Visual Studio Code version: 1.87.2
• Java extension version: 1.28.1

Steps To Reproduce

1. Observe that the project in the directory before/ can be successfully compiled with mvn --also-make --batch-mode without any warnings.
2. Start code before/project.code-workspace
3. Navigate to parentB/child3/src/main/java/com/example/Qux.java
4. Put the caret on the declaration of Bar b, right-click and select Go to definition.
5. The file Bar.java opens
6. Observe that the project in the directory after/ can be successfully compiled with mvn --also-make --batch-mode without any warnings.
7. Start code after/project.code-workspace
8. Navigate to parentB/child3/src/main/java/com/example/Qux.java
9. Put the caret on the declaration of Bar b.
10. Notice that it now gives the problem "Bar cannot be resolved to a type".

Current Result

In step 10, the class Bar cannot be found.

Expected Result

Step 10 should be like step 5, that the file Bar.java was opened when Go to definition was invoked.

Additional Information

issue-3544.zip

Start by unpacking issue-3544.zip and comparing the before/ and after/ Maven workspace projects. Run the listed mvn commands, then reproduce Go to definition from Qux.java for Bar in each workspace. Done means the after/ project resolves Bar and opens Bar.java like before/.