#932 Create class collector and use it where applicable

- Extract logic for walking through a directory and loading its classes into a separate class
- Replace all implementations with the new ClassCollector

src/test/java/fr/xephi/authme/TestHelper.java

@@ -28,6 +28,8 @@ import static org.mockito.Mockito.verify;
  */
 public final class TestHelper {
 
+    public static final String SOURCES_FOLDER = "src/main/java/";
+    public static final String TEST_SOURCES_FOLDER = "src/test/java/";
     public static final String PROJECT_ROOT = "/fr/xephi/authme/";
 
     private TestHelper() {