[wip] package-loader: Implement a common package loader mechanism
This also demonstrate the following reimplementation of:
- gnu
- pypi
- npm (not yet)
Note:
- This will get split into multiple diffs.
- This will also probably get moved out of swh-loader-core.
- In the mean time, to demonstrate progress has been done, here is the wip diff.
- Also, this allows to check everything is fine regarding the ci
Related T1389
Test Plan
tox
Migrated from D2028 (view on Phabricator)