Omit archive file_name during conversion

[rogierx]

Summary

Fixes #500 by omitting V0 source.fn when converting URL-based archive sources to V1 recipes.

fn was previously moved to file_name, but file_name is not valid for archive source URLs in rattler-build recipes. The converter now detects archive source URLs, removes fn, and emits a warning so users know the filename override could not be translated directly.

Validation

• PYTHONPATH=/tmp/crm-test-deps:$PWD /opt/anaconda3/bin/python -m pytest tests/parser/test_recipe_parser_convert.py -q -k 'boto or pytest_pep8 or pytest-pep8'
• PYTHONPATH=/tmp/crm-test-deps:$PWD /opt/anaconda3/bin/python -m pytest tests/parser/test_recipe_parser_convert.py -q

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