Map win32gui imports to pywin32

[StantonMatt]

Fixes #331.

win32gui is provided by the pywin32 distribution, so requirements generated from projects importing win32gui should use pywin32 instead of the import name. This adds the mapping and extends the existing package-name mapping test to cover it.

Verification:

• .venv/bin/python -m unittest tests.test_pipreqs.TestPipreqs.test_get_pkg_names
• .venv/bin/python -m unittest discover -s tests
• .venv/bin/python -m flake8 pipreqs tests
• git diff --check
• review-fix-loop: no blocking correctness issues

[bubaizhanshen]

I checked this locally on Python 3.10.

get_pkg_names(['win32gui']) now returns ['pywin32'], which matches the package users need for the import in #331.

Checks I ran:

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

All passed.