Remove non-mutated code from sys.path

[Otto-AA]

This change should help troubleshooting errors, where the package setup in ./mutants is not correct.

Currently, when the tests in ./mutants/ try to import a package that is not in ./mutants/, it will also look in the original (non-mutated) source code. Is there ever a case, where this is desired? I would guess that also_copy covers these use cases, but I am not sure, so I wanted to ask for your opinion @boxed .

The reason I want to prevent importing the original code is that it hides the error cause if the package setup of ./mutants is wrong. If the package is not setup properly (e.g. #370 , which only has a single file for paths_to_mutate rather than the whole source), the tests currently just use the original code and thus always pass. We only stop at the forced fail test.

With these changes, it will already fail at the test collecting stage. If debug=true, we also have the error cause reported:

ImportError while loading conftest '/.../narwhals/mutants/tests/conftest.py'.
../tests/conftest.py:14: in <module>
    from narwhals.utils import generate_temporary_column_name
E   ModuleNotFoundError: No module named 'narwhals.utils'
    exit code 4

[boxed]

I like it!