warn on ambiguous -p plugin module usage

[kartikdp]

Summary

-p <name> can silently do the wrong thing with --disable-plugin-autoload
when <name> is an importable top-level module that has no pytest hooks,
while the real plugin is exposed via a pytest11 entry-point submodule.

This change adds a focused warning in that case:

• if the imported module has no pytest hooks
• and a pytest11 entry-point exists in one of its submodules
• pytest emits PytestConfigWarning suggesting the entry-point name (for
  example -p recording)

closes #14135

Impact

This improves correctness and UX for plugin selection, especially in autoload-
disabled runs, by surfacing likely misconfiguration instead of failing
silently.

How tested

• Added regression test:
  • testing/ test_config.py::test_disable_plugin_autoload_warns_for_submodule_entrypoint
• Ran focused tests:
  • uv run -m pytest testing/test_config.py -k "disable_plugin_autoload_warns_for_submodule_entrypoint or test_disable_plugin_autoload"
  • uv run -m pytest testing/test_pluginmanager.py -k "import_plugin_importname or import_plugin_dotted_name"
• Ran formatting/lint/hooks on changed files:
  • uv run --with pre-commit pre-commit run --files src/_pytest/config/ __init__.py testing/test_config.py changelog/14135.bugfix.rst

Checklist

• Include documentation when adding new features. (Not applicable: bugfix,
  no new feature)
• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits.
• Add text like closes #XYZW to PR description and/or commits.
• If AI agents were used, they are credited in Co-authored-by commit
  trailers.
• Create a new changelog file in the changelog directory.
• Add yourself to AUTHORS in alphabetical order. (Not needed for this
  change)