fixtures: match fixtures based on actual node hierarchy, not textual nodeids

Refs #11662.

--- Problem

Each fixture definition has a "visibility", the `FixtureDef.baseid`
attribute. This is nodeid-like string. When a certain `node` requests a
certain fixture name, we match node's nodeid against the fixture
definitions with this name.

The matching currently happens on the *textual* representation of the
nodeid - we split `node.nodeid` to its "parent nodeids" and then check
if the fixture's `baseid` is in there.

While this has worked so far, we really should try to avoid textual
manipulation of nodeids as much as possible. It has also caused problem
in an odd case of a `Package` in the root directory: the `Package` gets
nodeid `.`, while a `Module` in it gets nodeid `test_module.py`. And
textually, `.` is not a parent of `test_module.py`.

--- Solution

Avoid this entirely by just checking the node hierarchy itself. This is
made possible by the fact that we now have proper `Directory` nodes
(`Dir` or `Package`) for the entire hierarchy.

Also do the same for `_getautousenames` which is a similar deal.

The `iterparentnodeids` function is no longer used and is removed.

testing/python/fixtures.py

@@ -1574,7 +1574,7 @@ class TestFixtureManagerParseFactories:
             """
             def test_hello(item, fm):
                 for name in ("fm", "hello", "item"):
-                    faclist = fm.getfixturedefs(name, item.nodeid)
+                    faclist = fm.getfixturedefs(name, item)
                     assert len(faclist) == 1
                     fac = faclist[0]
                     assert fac.func.__name__ == name
@@ -1598,7 +1598,7 @@ class TestFixtureManagerParseFactories:
                 def hello(self, request):
                     return "class"
                 def test_hello(self, item, fm):
-                    faclist = fm.getfixturedefs("hello", item.nodeid)
+                    faclist = fm.getfixturedefs("hello", item)
                     print(faclist)
                     assert len(faclist) == 3
 
@@ -1804,7 +1804,7 @@ class TestAutouseDiscovery:
             """
             from _pytest.pytester import get_public_names
             def test_check_setup(item, fm):
-                autousenames = list(fm._getautousenames(item.nodeid))
+                autousenames = list(fm._getautousenames(item))
                 assert len(get_public_names(autousenames)) == 2
                 assert "perfunction2" in autousenames
                 assert "perfunction" in autousenames