-p option now can be used to early-load plugins by entry-point name

Fixes #4718

src/_pytest/config/__init__.py

@@ -497,7 +497,7 @@ class PytestPluginManager(PluginManager):
             if not name.startswith("pytest_"):
                 self.set_blocked("pytest_" + name)
         else:
-            self.import_plugin(arg)
+            self.import_plugin(arg, consider_entry_points=True)
 
     def consider_conftest(self, conftestmodule):
         self.register(conftestmodule, name=conftestmodule.__file__)
@@ -513,7 +513,11 @@ class PytestPluginManager(PluginManager):
         for import_spec in plugins:
             self.import_plugin(import_spec)
 
-    def import_plugin(self, modname):
+    def import_plugin(self, modname, consider_entry_points=False):
+        """
+        Imports a plugin with ``modname``. If ``consider_entry_points`` is True, entry point
+        names are also considered to find a plugin.
+        """
         # most often modname refers to builtin modules, e.g. "pytester",
         # "terminal" or "capture".  Those plugins are registered under their
         # basename for historic purposes but must be imported with the
@@ -524,22 +528,26 @@ class PytestPluginManager(PluginManager):
         modname = str(modname)
         if self.is_blocked(modname) or self.get_plugin(modname) is not None:
             return
-        if modname in builtin_plugins:
-            importspec = "_pytest." + modname
-        else:
-            importspec = modname
+
+        importspec = "_pytest." + modname if modname in builtin_plugins else modname
         self.rewrite_hook.mark_rewrite(importspec)
+
+        if consider_entry_points:
+            loaded = self.load_setuptools_entrypoints("pytest11", name=modname)
+            if loaded:
+                return
+
         try:
             __import__(importspec)
         except ImportError as e:
-            new_exc_type = ImportError
             new_exc_message = 'Error importing plugin "%s": %s' % (
                 modname,
                 safe_str(e.args[0]),
             )
-            new_exc = new_exc_type(new_exc_message)
+            new_exc = ImportError(new_exc_message)
+            tb = sys.exc_info()[2]
 
-            six.reraise(new_exc_type, new_exc, sys.exc_info()[2])
+            six.reraise(ImportError, new_exc, tb)
 
         except Skipped as e:
             from _pytest.warnings import _issue_warning_captured

src/_pytest/helpconfig.py

@@ -60,7 +60,7 @@ def pytest_addoption(parser):
         dest="plugins",
         default=[],
         metavar="name",
-        help="early-load given plugin (multi-allowed). "
+        help="early-load given plugin module name or entry point (multi-allowed). "
         "To avoid loading of plugins, use the `no:` prefix, e.g. "
         "`no:doctest`.",
     )