Map common import names to their package names

[bubaizhanshen]

Fixes #491. Addresses part of #360.

This maps common import names to their package names for kafka, janitor, skimage, and sklearn, and adds regression coverage for those aliases.

Checks:

• python -m unittest tests.test_pipreqs.TestPipreqs.test_get_pkg_names tests.test_pipreqs.TestPipreqs.test_get_pkg_names_maps_common_import_aliases
• python -m py_compile pipreqs/pipreqs.py tests/test_pipreqs.py
• flake8 tests/test_pipreqs.py
• git diff --check

[StantonMatt]

I checked this branch locally. The mapping changes are consistent with the existing import-to-distribution override path, and the new test covers the four aliases directly.

Validation from a project-local Python 3.9 venv:

• PYTHONPATH=. python -m unittest tests.test_pipreqs.TestPipreqs.test_get_pkg_names tests.test_pipreqs.TestPipreqs.test_get_pkg_names_maps_common_import_aliases -> 2 tests, OK
• PYTHONPATH=. python -m py_compile pipreqs/pipreqs.py tests/test_pipreqs.py -> OK
• python -m flake8 tests/test_pipreqs.py -> OK
• git diff --check origin/master...HEAD -> OK

Only local note: the unittest command emitted urllib3's LibreSSL warning from this macOS Python; it was not from this patch.