setup.py: use native implicit namespace package (#78773)

As specified on https://packaging.python.org/en/latest/guides/packaging-namespace-packages/

setup.py

@@ -7,7 +7,7 @@ import subprocess
 from distutils.cmd import Command
 from distutils.command.build import build as _build
 from distutils.command.sdist import sdist as _sdist
-from setuptools import setup, find_packages
+from setuptools import setup, find_namespace_packages
 from xstatic.pkg import godo as xs
 
 
@@ -89,11 +89,7 @@ setup(
     license=xs.LICENSE,
     url=xs.HOMEPAGE,
     platforms=xs.PLATFORMS,
-    packages=find_packages(),
-    namespace_packages=[
-        'xstatic',
-        'xstatic.pkg',
-    ],
+    packages=find_namespace_packages(include=['xstatic.*', 'xstatic.pkg.*']),
     include_package_data=True,
     package_data={
         'xstatic.pkg.godo': ['data/*'],

xstatic/__init__.py

@@ -1 +0,0 @@
-__import__('pkg_resources').declare_namespace(__name__)

xstatic/pkg/__init__.py

@@ -1 +0,0 @@
-__import__('pkg_resources').declare_namespace(__name__)