Fix #9608: apidoc: module is not described if implicit namespace package

To make implicit namespace packages referencable, this outputs empty
module definitions for them.

Author: tk0miya

CHANGES

@@ -25,6 +25,8 @@ Features added
 Bugs fixed
 ----------
 
+* #9608: apidoc: apidoc does not generate a module definition for implicit
+  namespace package
 * #9487: autodoc: typehint for cached_property is not shown
 * #9509: autodoc: AttributeError is raised on failed resolving typehints
 * #9518: autodoc: autodoc_docstring_signature does not effect to ``__init__()``

sphinx/templates/apidoc/package.rst_t

@@ -19,6 +19,10 @@
 {{- [pkgname, "package"] | join(" ") | e | heading }}
 {% endif %}
 
+{%- if is_namespace %}
+.. py:module:: {{ pkgname }}
+{% endif %}
+
 {%- if modulefirst and not is_namespace %}
 {{ automodule(pkgname, automodule_options) }}
 {% endif %}

tests/test_ext_apidoc.py

@@ -635,6 +635,8 @@ def test_namespace_package_file(tempdir):
     assert content == ("testpkg namespace\n"
                        "=================\n"
                        "\n"
+                       ".. py:module:: testpkg\n"
+                       "\n"
                        "Submodules\n"
                        "----------\n"
                        "\n"