pytest-plugin: move the plugin in a dedicated non-swh module

This is in fact required for the pytest plugin loading mecanism to work
without hijacking the loading of swh packages.
See https://github.com/pytest-dev/pytest/issues/2042 for more details.

setup.py

@@ -46,6 +46,7 @@ setup(
     author_email='swh-devel@inria.fr',
     url='https://forge.softwareheritage.org/diffusion/DCORE/',
     packages=find_packages(),
+    py_modules=['pytest_swh_core'],
     scripts=[],
     install_requires=parse_requirements(None, 'swh'),
     setup_requires=['vcversioner'],
@@ -63,8 +64,8 @@ setup(
         [swh.cli.subcommands]
         db=swh.core.cli.db:db
         db-init=swh.core.cli.db:db_init
-        # [pytest11]
-        # pytest_swh_core = swh.core.pytest_plugin
+        [pytest11]
+        pytest_swh_core = pytest_swh_core
     ''',
     classifiers=[
         "Programming Language :: Python :: 3",

swh/core/tests/test_pytest_plugin.py

@@ -7,7 +7,7 @@ import requests
 
 from os import path
 
-from swh.core.pytest_plugin import local_get_factory
+from pytest_swh_core import local_get_factory
 
 
 def test_get_response_cb_with_visits_nominal(local_get_visits):