Fixes #58 Implement egg-info version parsing

[arnowaschk]

Fixes #58

pipreqs could misparse <package>.egg-info metadata directories and emit malformed package/version pairs. This patch:

• prefers METADATA / PKG-INFO when available
• preserves the old version fallback for regular directory names
• adds regression coverage for egg-info and metadata-based parsing

I verified the focused regression tests locally.

Greetings Arno

[StantonMatt]

I checked this locally against #58. The branch fixes the original no-version <package>.egg-info case in a way that keeps the package name usable instead of splitting the suffix as a bogus version.

Direct reproduction:

origin/master: [{'name': 'mypackage.egg', 'version': 'info', 'exports': ['mypackage']}]
pr-529:       [{'name': 'mypackage', 'version': None, 'exports': ['mypackage']}]

I also checked the metadata path and the old directory-name fallback through the new tests. Local verification on Python 3.9.6:

python -m unittest tests.test_pipreqs.TestPipreqs.test_get_locally_installed_packages_ignores_egg_info_suffix tests.test_pipreqs.TestPipreqs.test_get_package_name_and_version_preserves_regular_version tests.test_pipreqs.TestPipreqs.test_get_package_name_and_version_keeps_hyphenated_name_without_version tests.test_pipreqs.TestPipreqs.test_get_locally_installed_packages_prefers_metadata_name
python -m unittest discover
python -m py_compile pipreqs/pipreqs.py tests/test_pipreqs.py
python -m flake8 pipreqs/pipreqs.py tests/test_pipreqs.py
git diff --check origin/master...HEAD

The focused tests passed, full discovery passed 33 tests, and lint/compile/diff checks passed. GitHub reports no check runs for this PR, so there is no upstream CI result to claim, but I do not see a blocking issue from this pass.

[arnowaschk]

@StantonMatt Thank you for your efforts!