gnu: Separate listing and parsing logic and add integration test
Also:
- drop the unused package name loader side (also it collides between gnu/old-gnu listing).
- Add more tests on the parsing side
- clean up docstrings
- Improve slightly the parsing code
Related T2023
Depends on !106 (closed)
Test Plan
tox
Migrated from D2076 (view on Phabricator)