core/loader: Fix get_save_data_path implementation

Related swh-loader-git#1987 (closed)

Also, unrelatedly:

• pep8: Fix log.warning calls

• pytest.ini: Remove warnings about our custom markers (see [1])

• package/loader: Fix mispelling

• [1] https://docs.pytest.org/en/latest/mark.html

Test Plan

tox

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Migrated from D1967 (view on Phabricator)