adjust the structure of detectors

mmdet/models/__init__.py

mmdet/models/backbones/resnet.py

mmdet/models/bbox_heads/bbox_head.py

mmdet/models/builder.py

mmdet/models/detectors/__init__.py

mmdet/models/detectors/base.py

mmdet/models/detectors/detector.py

mmdet/models/detectors/faster_rcnn.py

mmdet/models/detectors/mask_rcnn.py

mmdet/models/detectors/rpn.py

mmdet/models/detectors/testing_mixins.py

mmdet/models/detectors/two_stage.py

mmdet/models/necks/fpn.py

mmdet/models/rpn_heads/rpn_head.py