Use the default_release endpoint to provide a default branch in the snapshots

Summary:
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 D425

Test Plan: The snapshot tests have been updated for the new default branch

Reviewers: ardumont

Differential Revision: https://forge.softwareheritage.org/D426