wheel: fix installation of empty namespace package

changelog.d/1861.change.rst

+Fix empty namespace package installation from wheel.

setuptools/tests/test_wheel.py

@@ -450,6 +450,34 @@ WHEEL_INSTALL_TESTS = (
         }),
     ),
 
+    dict(
+        id='empty_namespace_package',
+        file_defs={
+            'foobar': {
+                '__init__.py': "__import__('pkg_resources').declare_namespace(__name__)",
+            },
+        },
+        setup_kwargs=dict(
+            namespace_packages=['foobar'],
+            packages=['foobar'],
+        ),
+        install_tree=flatten_tree({
+            'foo-1.0-py{py_version}.egg': [
+                'foo-1.0-py{py_version}-nspkg.pth',
+                {'EGG-INFO': [
+                    'PKG-INFO',
+                    'RECORD',
+                    'WHEEL',
+                    'namespace_packages.txt',
+                    'top_level.txt',
+                ]},
+                {'foobar': [
+                    '__init__.py',
+                ]},
+            ]
+        }),
+    ),
+
     dict(
         id='data_in_package',
         file_defs={

setuptools/wheel.py

@@ -213,6 +213,8 @@ class Wheel:
             for mod in namespace_packages:
                 mod_dir = os.path.join(destination_eggdir, *mod.split('.'))
                 mod_init = os.path.join(mod_dir, '__init__.py')
-                if os.path.exists(mod_dir) and not os.path.exists(mod_init):
+                if not os.path.exists(mod_dir):
+                    os.mkdir(mod_dir)
+                if not os.path.exists(mod_init):
                     with open(mod_init, 'w') as fp:
                         fp.write(NAMESPACE_PACKAGE_INIT)