make setuptools happy, fixes #6874

work around setuptools puking about:

          ############################
          # Package would be ignored #
          ############################
          Python recognizes 'borg.cache_sync' as an importable package,
          but it is not listed in the `packages` configuration of setuptools.

          'borg.cache_sync' 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 'borg.cache_sync' 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.

setup.py

@@ -11,7 +11,7 @@ except ImportError:
     multiprocessing = None
 
 from setuptools.command.build_ext import build_ext
-from setuptools import setup, find_packages, Extension, Command
+from setuptools import setup, find_namespace_packages, Extension, Command
 from setuptools.command.sdist import sdist
 
 try:
@@ -292,7 +292,7 @@ setup(
         'Topic :: Security :: Cryptography',
         'Topic :: System :: Archiving :: Backup',
     ],
-    packages=find_packages('src'),
+    packages=find_namespace_packages('src'),
     package_dir={'': 'src'},
     zip_safe=False,
     entry_points={