Deduplicate generated requirements

[StantonMatt]

Summary

• Deduplicate generated requirement entries by normalized package name before writing output
• Keep the first resolved versioned entry, while allowing a later versioned alias to replace an unpinned one
• Leave --diff and --clean raw-name comparisons unchanged so existing requirement spellings are preserved

Context

Addresses the duplicate-output part of #504, including cases such as PyYAML/pyyaml and repeated local package records.

Verification

• .venv/bin/python -m unittest tests.test_pipreqs.TestPipreqs.test_deduplicate_imports_uses_normalized_package_names tests.test_pipreqs.TestPipreqs.test_init_deduplicates_normalized_requirements tests.test_pipreqs.TestPipreqs.test_init_clean_keeps_normalized_alias_requirements
• .venv/bin/python -m unittest discover
• uv run --with flake8 flake8 pipreqs tests
• git diff --check
• review-fix-loop: no actionable correctness issues