Reorganize docker.transport package

Signed-off-by: Joffrey F <joffrey@docker.com>

docker/transport/__init__.py

 # flake8: noqa
-from .npipe import NpipeAdapter
 from .unixconn import UnixAdapter
+try:
+    from .npipeconn import NpipeAdapter
+except ImportError:
+    pass
\ No newline at end of file

docker/transport/npipe/__init__.py

-from .npipeconn import NpipeAdapter  # flake8: noqa

docker/transport/unixconn/__init__.py

-from .unixconn import UnixAdapter  # flake8: noqa