Prevent root conftest.py from loading swh.core at module loading time

move the import statement in swhmain() body.
Having the swh.core module loaded with the execution of conftest.py can
have weird side effects, especially on pytest's discovery mechanism in
the context of tox, namely make pytest load test files from the source
directory rather than from the installed package in the tox venv,
potentially leading to ImportPathMismatchError.