Expand default ignored directories

[StantonMatt]

Summary

• centralize the default scanner ignore list for reuse
• ignore common generated/cache/environment directories such as .mypy_cache, .nox, build, dist, node_modules, and pypackages by default
• add coverage that files inside default ignored directories do not contribute imports

Fixes #420.

Verification

• .venv/bin/python -m unittest tests.test_pipreqs.TestPipreqs.test_get_all_imports_skips_default_ignored_directories
• .venv/bin/python -m unittest tests.test_pipreqs
• .venv/bin/python -m flake8 pipreqs tests
• review-fix-loop: clean fresh review, no actionable correctness issues

[bubaizhanshen]

I checked this locally on Python 3.10.

The new default-ignore path works for the issue shape from #469/#495: with main.py importing requests and a .venv/lib/bad.py file containing undecodable bytes plus import click, get_all_imports() returns only ['requests']. So the scanner no longer reads into the virtualenv by default.

Checks I ran:

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

All passed. I did not use test_ignored_directory as a signal here because it waits on a live PyPI request in this environment, but the default directory-pruning behavior changed by this PR looks good from this pass.