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.
Ran Benita