PyCollector._genfunctions: use already created fixtureinfo (#6636)

`Function` creates a `_fixtureinfo` already:
https://github.com/pytest-dev/pytest/blob/fed535694/src/_pytest/python.py#L1392-L1395

Author: blueyed

src/_pytest/python.py

@@ -390,7 +390,7 @@ def _genfunctions(self, name, funcobj):
         fm = self.session._fixturemanager
 
         definition = FunctionDefinition(name=name, parent=self, callobj=funcobj)
-        fixtureinfo = fm.getfixtureinfo(definition, funcobj, cls)
+        fixtureinfo = definition._fixtureinfo
 
         metafunc = Metafunc(
             definition, fixtureinfo, self.config, cls=cls, module=module