tests: Use swh.core's requests_mock_datadir fixture

MANIFEST.in

requirements-swh.txt

requirements-test.txt

swh/loader/package/tests/conftest.py

swh/loader/package/tests/fixture/__init__.py

swh/loader/package/tests/fixture/conftest.py

swh/loader/package/tests/fixture/data/example.com/file.json

swh/loader/package/tests/fixture/test_conftest.py

swh/loader/package/tests/test_conftest.py

swh/loader/package/tests/test_deposit.py

swh/loader/package/tests/test_gnu.py

swh/loader/package/tests/test_npm.py

swh/loader/package/tests/test_pypi.py

tox.ini