Replace in-tree test fixture with per-dir conftest

Pytest automatically load `conftest.py` files from directory tests,
so it is not necessary to use an in-tree pytest plugin for that.

This issue was considered a blocker in
pypa#2318.

Author: abravalheri

conftest.py

@@ -3,9 +3,6 @@
 import pytest
 
 
-pytest_plugins = 'setuptools.tests.fixtures'
-
-
 def pytest_addoption(parser):
     parser.addoption(
         "--package_name", action="append", default=[],