This is available when the argument is a `--pyargs` argument (resolved
from a python module path). Will be used in an upcoming commit.

No logical change, preparation for the next commit.

…llection arguments

(diff better viewed ignoring whitespace)

In pytest<8, the collection tree for `pyargs` arguments in an invocation
like this:

    pytest --collect-only --pyargs pyflakes.test.test_undefined_names

looked like this:

```
<Package test>
  <Module test_undefined_names.py>
    <UnitTestCase Test>
      <TestCaseFunction test_annotationUndefined>
      ... snipped ...
```

The pytest 8 collection improvements changed it to this:

```
<Dir pytest>
  <Dir .tox>
    <Dir venv>
      <Dir lib>
        <Dir python3.11>
          <Dir site-packages>
            <Package pyflakes>
              <Package test>
                <Module test_undefined_names.py>
                  <UnitTestCase Test>
                    <TestCaseFunction test_annotationUndefined>
                    ... snipped ...
```

Besides being egregious (and potentially even worse than the above,
going all the way to the root, for system-installed packages, as is
apparently common in CI), this also caused permission errors when trying
to probe some of those intermediate directories.

This change makes `--pyargs` arguments no longer try to add parent
directories to the collection tree according to the `--confcutdir` like
they're regular arguments. Instead, only add the parents that are in the
import path. This now looks like this:

```
<Package .tox/venv/lib/python3.11/site-packages/pyflakes>
  <Package test>
    <Module test_undefined_names.py>
      <UnitTestCase Test>
        <TestCaseFunction test_annotationUndefined>
        ... snipped ...
```

Fix pytest-dev#11904.