Protect custom savepath from accidental overwrite

[StantonMatt]

Summary

• apply the existing overwrite protection to the resolved output path, including custom --savepath files
• keep --print, --diff, --clean, and stdout output from being blocked by the file-generation guard
• add regression coverage for custom savepath no-force, force, diff, and clean behavior

Fixes #336.

Verification

• .venv/bin/python -m unittest tests.test_pipreqs.TestPipreqs.test_init_savepath tests.test_pipreqs.TestPipreqs.test_init_overwrite tests.test_pipreqs.TestPipreqs.test_init_savepath_does_not_overwrite_existing_file_without_force tests.test_pipreqs.TestPipreqs.test_init_savepath_force_overwrites_existing_file tests.test_pipreqs.TestPipreqs.test_init_savepath_existing_file_does_not_block_diff tests.test_pipreqs.TestPipreqs.test_init_savepath_existing_file_does_not_block_clean
• .venv/bin/python -m unittest discover
• uv run --with flake8 flake8 pipreqs tests
• git diff --check
• review-fix-loop clean