Handle -p plug after -p no:plug.

This can be used to override a blocked plugin (e.g. in "addopts") from the
command line etc.

src/_pytest/config/__init__.py

@@ -496,6 +496,14 @@ class PytestPluginManager(PluginManager):
             if not name.startswith("pytest_"):
                 self.set_blocked("pytest_" + name)
         else:
+            name = arg
+            # Unblock the plugin.  None indicates that it has been blocked.
+            # There is no interface with pluggy for this.
+            if self._name2plugin.get(name, -1) is None:
+                del self._name2plugin[name]
+            if not name.startswith("pytest_"):
+                if self._name2plugin.get("pytest_" + name, -1) is None:
+                    del self._name2plugin["pytest_" + name]
             self.import_plugin(arg, consider_entry_points=True)
 
     def consider_conftest(self, conftestmodule):