The :globalvar:`pytest_plugins` variable and the :envvar:`PYTEST_PLUGINS` environment variable now also accept entry point names of installed plugins (e.g. ``xdist``), in addition to importable module names, matching the behavior of the :option:`-p` command-line option.

As part of this change, plugins specified by entry point name through these mechanisms are now also listed in the ``plugins:`` line of the terminal header (:issue:`12615`).

Based on the original implementation by Marcel Telka in :pr:`12616`.
