Support multiple Java modules per POM with Maven 4 Module Source Hierarchy

[ascheman]

Follow-up to #3345 / #3392, which add support for the Maven 4 Module Source Hierarchy with ONE Java module per POM.

Maven 4 allows declaring several modules in one POM (<build><sources> with multiple <module> entries, POM model 4.1.0). maven-compiler-plugin 4.x compiles them to target/classes/<module-1>/, target/classes/<module-2>/, … and test classes to target/test-classes/<module-N>/.

With the single-module support from #3392, surefire on such a project:

1. detects only the FIRST nested module descriptor,
2. scans only that module's nested test directory,
3. decides classpath-vs-module-path placement of dependencies with that single module descriptor via LocationManager.resolvePaths(...) — dependencies that only sibling modules require stay on the classpath, and the forked JVM fails at boot:
   java.lang.module.FindException: Module <dep> not found, required by <sibling-module>
   (all nested modules resolve into the boot layer because target/classes is on the module path as a directory of modules, together with --add-modules ALL-MODULE-PATH),
4. patches and opens only the first module.

Reproduced with a real-world 3-module component (jakarta.json/jsonb implementation, 602 tests): zero-config surefire fails with the FindException above; the only working setup is one surefire execution per module with explicit classesDirectory/testClassesDirectory/additionalClasspathElements.

Expected: one mvn test execution runs the tests of ALL modules — each module patched with its own target/test-classes/<module>/, each opened for reflection, and the dependency split computed as the union over all module descriptors.

A PR implementing this (stacked on #3392) follows immediately.

cc @desruisseaux — this is the multi-module follow-up announced in #3345.