I think this named function makes the code a bit easier to understand.

Also change the check to explicitly check for "is a sub-path of" instead
of the previous check which only worked assuming that path is within
confcutdir or a direct parent of it.

The tests patch `os.path.abspath` which can break some pytest internal
code since the patching is not undone immediately.

Nothing should mutate the internal data structure here.

This became the wrong thing to do since
322190f.

It duplicates what PluginManager already knows, and no longer needed now
that symlinks are not resolved (see previous commit).