Use functools.lru_cache with _getconftest_pathlist

For pytest's own suite the `cache_info()` looks as follows:

    > session.config._getconftest_pathlist.cache_info()
    CacheInfo(hits=231, misses=19, maxsize=None, currsize=19)

While it does not really make a difference for me this might help with
larger test suites / the case mentioned in
https://github.com/pytest-dev/pytest/issues/2206#issuecomment-432623646.

src/_pytest/config/__init__.py

@@ -1,6 +1,7 @@
 """ command line options, ini-file and conftest.py processing. """
 from __future__ import absolute_import, division, print_function
 import argparse
+import functools
 import inspect
 import shlex
 import types
@@ -893,6 +894,7 @@ class Config(object):
             assert type is None
             return value
 
+    @functools.lru_cache(maxsize=None)
     def _getconftest_pathlist(self, name, path):
         try:
             mod, relroots = self.pluginmanager._rget_with_confmod(name, path)