Use the default_release endpoint to provide a default branch in the snapshots
The release provided by the PyPI API is the default version that is presented by the web interface, as well as the one that is used for pip install. It makes sense to present it as the default branch in the archive.
Depends on !11 (closed)
Test Plan
The snapshot tests have been updated for the new default branch
Migrated from D426 (view on Phabricator)