Keep requests package mapping lowercase

[StantonMatt]

Summary

• update the requests import mapping to emit the lowercase PyPI package name
• add a regression test for get_pkg_names(['requests'])

Fixes #500.

Verification

• uv run --with-editable . python -m unittest tests.test_pipreqs.TestPipreqs.test_get_pkg_names_keeps_requests_lowercase
• uv run --with flake8 python -m unittest discover
• uv run --with flake8 flake8 pipreqs tests
• git diff --check
• review-fix-loop clean