Consider entry_points for PYTEST_PLUGINS and pytest_plugins

This fixes two issues related to the `PYTEST_PLUGINS` environment variable and
`pytest_plugins` global variable:

* plugins loaded this way are not listed in the list of plugins,
* it is not possible to load plugins using their names in `entry_points.txt`
  files.

Fixes #12615.

Author: mtelka

Diff for: AUTHORS

@@ -257,6 +257,7 @@ Mandeep Bhutani
 Manuel Krebber
 Marc Mueller
 Marc Schlaich
+Marcel Telka
 Marcelo Duarte Trevisani
 Marcin Bachry
 Marc Bresson

Diff for: changelog/12615.bugfix.rst

@@ -0,0 +1,5 @@
+Plugins specified in the :globalvar:`pytest_plugins` config setting and
+:envvar:`PYTEST_PLUGINS` environment variable now allow using
+:ref:`entry points <pip-installable plugins>` names additionally to the
+importable definitions. Prior to this release, these identifiers used to only
+work with the ``-p`` CLI option. -- by :user:`mtelka`.

Diff for: doc/en/reference/reference.rst

@@ -1153,7 +1153,7 @@ specified plugins will be loaded.
 
 .. envvar:: PYTEST_PLUGINS
 
-Contains comma-separated list of modules that should be loaded as plugins:
+Contains comma-separated list of modules or entry points that should be loaded as plugins:
 
 .. code-block:: bash
 

Diff for: src/_pytest/config/__init__.py

@@ -828,7 +828,7 @@ def _import_plugin_specs(
     ) -> None:
         plugins = _get_plugin_specs_as_list(spec)
         for import_spec in plugins:
-            self.import_plugin(import_spec)
+            self.import_plugin(import_spec, consider_entry_points=True)
 
     def import_plugin(self, modname: str, consider_entry_points: bool = False) -> None:
         """Import a plugin with ``modname``.