Add swh.storage.api package to setup.py

setup.py

@@ -21,7 +21,7 @@ setup(
     author='Software Heritage developers',
     author_email='swh-devel@inria.fr',
     url='https://forge.softwareheritage.org/diffusion/DSTO/',
-    packages=['swh.storage', 'swh.storage.tests'],
+    packages=['swh.storage', 'swh.storage.api', 'swh.storage.tests'],
     scripts=[
         'bin/swh-objstorage-add-dir',
         'bin/swh-objstorage-fsck',