fixtures: use the item `fixturenames` in `request.fixturenames`

`_pyfuncitem.fixturenames` is just an alias for
`_pyfuncitem._fixtureinfo.names_closure` (at least in core pytest), so
let's do the less abstraction-breaking thing.

src/_pytest/fixtures.py

@@ -405,7 +405,7 @@ class FixtureRequest(abc.ABC):
     @property
     def fixturenames(self) -> List[str]:
         """Names of all active fixtures in this request."""
-        result = list(self._pyfuncitem._fixtureinfo.names_closure)
+        result = list(self._pyfuncitem.fixturenames)
         result.extend(set(self._fixture_defs).difference(result))
         return result