This replaces !19 (closed), which was a first attempt to fix pypi caching (as written in a few places, the idea is to stop caching indexes).

This depends on:

• slapos.libnetworkcache@1130d381 and slapos.libnetworkcache@8a582cc3 (not merged yet)
• erp5@1c6f18bf (not merged but already applied on www.shacache.org)

This invalidates all PyPI cache entries, and only that.