shift pytest_configure/unconfigure/addoption/namespace hook calling to config object.

The _pytest.config module itself is no longer a plugin but the actual
config instance is plugin-registered as ``pytestconfig``.
This allows to put most pytest specific logic to _pytest.config instead
of in the core pluginmanager.

_pytest/pytester.py

@@ -390,9 +390,9 @@ class TmpTestdir:
 
     def parseconfigure(self, *args):
         config = self.parseconfig(*args)
-        config.pluginmanager.do_configure(config)
+        config.do_configure()
         self.request.addfinalizer(lambda:
-        config.pluginmanager.do_unconfigure(config))
+        config.do_unconfigure())
         return config
 
     def getitem(self,  source, funcname="test_func"):