doctest: fix autouse fixtures possibly not getting picked up

Fix #11929.

Figured out what's going on. We have the following collection tree:

```
<Dir pyspacewar>
  <Dir src>
    <Package pyspacewar>
      <Package tests>
        <DoctestModule test_main.py>
          <DoctestItem pyspacewar.tests.test_main.doctest_main>
```

And the `test_main.py` contains an autouse fixture (`fake_game_ui`) that
`doctest_main` needs in order to run properly. The fixture doesn't run!
It doesn't run because nothing collects the fixtures from (calls
`parsefactories()` on) the `test_main.py` `DoctestModule`.

How come it only started happening with commit
ab63ebb3dc07b89670b96ae97044f48406c44fa0? Turns out it mostly only
worked accidentally. Each `DoctestModule` is also collected as a normal
`Module`, with the `Module` collected after the `DoctestModule`. For
example, if we add a non-doctest test to `test_main.py`, the collection
tree looks like this:

```
<Dir pyspacewar>
  <Dir src>
    <Package pyspacewar>
      <Package tests>
        <DoctestModule test_main.py>
          <DoctestItem pyspacewar.tests.test_main.doctest_main>
        <Module test_main.py>
          <Function test_it>
```

Now, `Module` *does* collect fixtures. When autouse fixtures are
collected, they are added to the `_nodeid_autousenames` dict.

Before ab63ebb3dc, `DoctestItem` consults
`_nodeid_autousenames` at *setup* time. At this point, the `Module` has
collected and so it ended up picking the autouse fixture (this relies on
another "accident", that the `DoctestModule` and `Module` have the same
node ID).

After ab63ebb3dc, `DoctestItem` consults
`_nodeid_autousenames` at *collection* time (= when it's created). At
this point, the `Module` hasn't collected yet, so the autouse fixture is
not picked out.

The fix is simple -- have `DoctestModule.collect()` call
`parsefactories`. From some testing I've done it shouldn't have negative
consequences (I hope).

changelog/11929.bugfix.rst

@@ -0,0 +1 @@
+Fix a regression in pytest 8.0.0 whereby autouse fixtures defined in a module get ignored by the doctests in the module.

src/_pytest/doctest.py

@@ -567,6 +567,10 @@ class DoctestModule(Module):
             else:
                 raise
 
+        # While doctests currently don't support fixtures directly, we still
+        # need to pick up autouse fixtures.
+        self.session._fixturemanager.parsefactories(self)
+
         # Uses internal doctest module parsing mechanism.
         finder = MockAwareDocTestFinder()
         optionflags = get_optionflags(self.config)

testing/test_doctest.py

@@ -1376,6 +1376,38 @@ class TestDoctestAutoUseFixtures:
         str(result.stdout.no_fnmatch_line("*FAILURES*"))
         result.stdout.fnmatch_lines(["*=== 1 passed in *"])
 
+    @pytest.mark.parametrize("scope", [*SCOPES, "package"])
+    def test_auto_use_defined_in_same_module(
+        self, pytester: Pytester, scope: str
+    ) -> None:
+        """Autouse fixtures defined in the same module as the doctest get picked
+        up properly.
+
+        Regression test for #11929.
+        """
+        pytester.makepyfile(
+            f"""
+            import pytest
+
+            AUTO = "the fixture did not run"
+
+            @pytest.fixture(autouse=True, scope="{scope}")
+            def auto(request):
+                global AUTO
+                AUTO = "the fixture ran"
+
+            def my_doctest():
+                '''My doctest.
+
+                >>> my_doctest()
+                'the fixture ran'
+                '''
+                return AUTO
+            """
+        )
+        result = pytester.runpytest("--doctest-modules")
+        result.assert_outcomes(passed=1)
+
 
 class TestDoctestNamespaceFixture:
     SCOPES = ["module", "session", "class", "function"]