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

[bubaizhanshen]

I checked the mapping change locally on Python 3.10.

The focused tests pass:

python -m unittest tests.test_pipreqs.TestPipreqs.test_get_pkg_names_keeps_requests_lowercase tests.test_pipreqs.TestPipreqs.test_get_pkg_names
python -m flake8 pipreqs tests
python -m py_compile pipreqs/pipreqs.py tests/test_pipreqs.py
git diff --check upstream/master...HEAD

A small end-to-end probe with import requests changes the generated no-pin output from Requests on upstream/master to lowercase requests on this branch, which is the issue #500 is asking for.

One separate note: in my environment there are two local requests records visible, so this branch alone can write two lowercase requests lines. That is the duplicate-output issue covered by #537 rather than a problem with this mapping change itself.