private void addSourceRoot(String outputDirectory) {
        if (testConfig) {
            project.addTestCompileSourceRoot(outputDirectory);
        } else {
            project.addCompileSourceRoot(outputDirectory);
        }
    }
Returns List of all def-files in all defFilesDirectories, including path.
@return The list of def-files.
config-class-plugin/src/main/java/com/yahoo/vespa/ConfigGenMojo.java