fixtures: remove special cases when deciding when pytest.fixture() is a direct decoration

pytest.fixture() can be used either as

    @pytest.fixture
    def func(): ...

or as

    @pytest.fixture()
    def func(): ...

or (while maybe not intended)

    func = pytest.fixture(func)

so it needs to inspect internally whether it got a function in the first
positional argument or not.

Previously, there were was oddity. In the following,

    func = pytest.fixture(func, autouse=True)
    # OR
    func = pytest.fixture(func, parms=['a', 'b'])

The result is as if `func` wasn't passed.

There isn't any reason for this special that I can understand, so remove
it.

changelog/7253.bugfix.rst

@@ -0,0 +1,3 @@
+When using ``pytest.fixture`` on a function directly, as in ``pytest.fixture(func)``,
+if the ``autouse`` or ``params`` arguments are also passed, the function is no longer
+ignored, but is marked as a fixture.

src/_pytest/fixtures.py

@@ -1152,13 +1152,15 @@ def fixture(
     if params is not None:
         params = list(params)
 
-    if fixture_function and params is None and autouse is False:
+    fixture_marker = FixtureFunctionMarker(
-        # direct decoration
+        scope=scope, params=params, autouse=autouse, ids=ids, name=name,
-        return FixtureFunctionMarker(scope, params, autouse, name=name)(
+    )
-            fixture_function
-        )
 
-    return FixtureFunctionMarker(scope, params, autouse, ids=ids, name=name)
+    # Direct decoration.
+    if fixture_function:
+        return fixture_marker(fixture_function)
+
+    return fixture_marker
 
 
 def yield_fixture(