Don't try to modify project source roots (#358)

The aggregate javadoc mojo was calling `addAll` directly on the list returned by `project.getCompileSourceRoots`. This bug was silently ignored by Maven 3, but results in an `UnsupportedOperationException` on Maven 4.

Do a defensive copy instead.

Author: oehme

src/main/java/org/apache/maven/plugins/javadoc/AbstractJavadocMojo.java

@@ -2201,7 +2201,7 @@ protected Collection<JavadocModule> getSourcePaths() throws MavenReportException
                     if (subProject != project) {
                         Collection<Path> additionalSourcePaths = new ArrayList<>();
 
-                        List<String> sourceRoots = getProjectSourceRoots(subProject);
+                        List<String> sourceRoots = new ArrayList<>(getProjectSourceRoots(subProject));
 
                         if (subProject.getExecutionProject() != null) {
                             sourceRoots.addAll(getExecutionProjectSourceRoots(subProject));