setup.py: Remove sdisk build warning about importable packages

This fixes the sdist build warnings about importable packages (swh.vault.sql,
swh.vault.sql.upgrades). [1]

This aligns with the swh.storage's setup.py.

[1] for package in swh.vault.sql swh.vault.sql.upgrades
```
    ############################
    # Package would be ignored #
    ############################
    Python recognizes '$package' as an importable package,
    but it is not listed in the `packages` configuration of setuptools.

    '$package' has been automatically added to the distribution only
    because it may contain data files, but this behavior is likely to change
    in future versions of setuptools (and therefore is considered deprecated).

    Please make sure that '$package' is included as a package by using
    the `packages` configuration field or the proper discovery methods
    (for example by using `find_namespace_packages(...)`/`find_namespace:`
    instead of `find_packages(...)`/`find:`).

    You can read more about "package discovery" and "data files" on setuptools
    documentation page.
```

Refs. swh/infra/sysadm-environment#5044

setup.py

 #!/usr/bin/env python3
-# Copyright (C) 2015-2020  The Software Heritage developers
+# Copyright (C) 2015-2023  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -7,7 +7,7 @@
 from io import open
 from os import path
 
-from setuptools import find_packages, setup
+from setuptools import find_namespace_packages, setup
 
 here = path.abspath(path.dirname(__file__))
 
@@ -44,7 +44,7 @@ setup(
     author="Software Heritage developers",
     author_email="swh-devel@inria.fr",
     url="https://forge.softwareheritage.org/diffusion/DVAU/",
-    packages=find_packages(),
+    packages=find_namespace_packages(include=["swh", "swh.*"]),
     install_requires=parse_requirements() + parse_requirements("swh"),
     setup_requires=["setuptools-scm"],
     use_scm_version=True,