main: only include package parents in collection tree for --pyargs collection 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 #11904.

changelog/11904.bugfix.rst

@@ -0,0 +1,3 @@
+Fixed a regression in pytest 8.0.0 that would cause test collection to fail due to permission errors when using ``--pyargs``.
+
+This change improves the collection tree for tests specified using ``--pyargs``, see :pull:`12043` for a comparison with pytest 8.0 and <8.

src/_pytest/main.py

@@ -846,6 +846,7 @@ class Session(nodes.Collector):
 
             argpath = collection_argument.path
             names = collection_argument.parts
+            module_name = collection_argument.module_name
 
             # resolve_collection_argument() ensures this.
             if argpath.is_dir():
@@ -854,11 +855,20 @@ class Session(nodes.Collector):
             paths = [argpath]
             # Add relevant parents of the path, from the root, e.g.
             #   /a/b/c.py -> [/, /a, /a/b, /a/b/c.py]
+            if module_name is None:
                 # Paths outside of the confcutdir should not be considered.
                 for path in argpath.parents:
                     if not pm._is_in_confcutdir(path):
                         break
                     paths.insert(0, path)
+            else:
+                # For --pyargs arguments, only consider paths matching the module
+                # name. Paths beyond the package hierarchy are not included.
+                module_name_parts = module_name.split(".")
+                for i, path in enumerate(argpath.parents, 2):
+                    if i > len(module_name_parts) or path.stem != module_name_parts[-i]:
+                        break
+                    paths.insert(0, path)
 
             # Start going over the parts from the root, collecting each level
             # and discarding all nodes which don't match the level's part.

testing/test_collection.py

@@ -1787,3 +1787,48 @@ def test_collect_short_file_windows(pytester: Pytester) -> None:
     test_file.write_text("def test(): pass", encoding="UTF-8")
     result = pytester.runpytest(short_path)
     assert result.parseoutcomes() == {"passed": 1}
+
+
+def test_pyargs_collection_tree(pytester: Pytester, monkeypatch: MonkeyPatch) -> None:
+    """When using `--pyargs`, the collection tree of a pyargs collection
+    argument should only include parents in the import path, not up to confcutdir.
+
+    Regression test for #11904.
+    """
+    site_packages = pytester.path / "venv/lib/site-packages"
+    site_packages.mkdir(parents=True)
+    monkeypatch.syspath_prepend(site_packages)
+    pytester.makepyfile(
+        **{
+            "venv/lib/site-packages/pkg/__init__.py": "",
+            "venv/lib/site-packages/pkg/sub/__init__.py": "",
+            "venv/lib/site-packages/pkg/sub/test_it.py": "def test(): pass",
+        }
+    )
+
+    result = pytester.runpytest("--pyargs", "--collect-only", "pkg.sub.test_it")
+    assert result.ret == ExitCode.OK
+    result.stdout.fnmatch_lines(
+        [
+            "<Package venv/lib/site-packages/pkg>",
+            "  <Package sub>",
+            "    <Module test_it.py>",
+            "      <Function test>",
+        ],
+        consecutive=True,
+    )
+
+    # Now with an unrelated rootdir with unrelated files.
+    monkeypatch.chdir(tempfile.gettempdir())
+
+    result = pytester.runpytest("--pyargs", "--collect-only", "pkg.sub.test_it")
+    assert result.ret == ExitCode.OK
+    result.stdout.fnmatch_lines(
+        [
+            "<Package *pkg>",
+            "  <Package sub>",
+            "    <Module test_it.py>",
+            "      <Function test>",
+        ],
+        consecutive=True,
+    )