swh.indexer.storage: Move storage relative code in subtree

swh/indexer/__init__.py

swh/indexer/storage/__init__.py

swh/indexer/storage/api/__init__.py

swh/indexer/storage/api/client.py

swh/indexer/storage/api/server.py

swh/indexer/storage/converters.py

swh/indexer/storage/db.py

swh/indexer/tests/common.py

swh/indexer/tests/storage/__init__.py

swh/indexer/tests/storage/test_api_client.py

swh/indexer/tests/storage/test_converters.py

swh/indexer/tests/storage/test_storage.py

swh/indexer/tests/test_utils.py