* Enable storage retention for pypi
PyPI names are stored the way purl normalizes them, lowercased and with _ turned into -, so Typing_Extensions and typing-extensions hit the same package. The mirror can't fetch PyPI files today, so there's no mirror test.
Refs #306
* Correct the pypi package rule note
Dots in PyPI names aren't normalized, so zope.interface and zope_interface files end up under two packages. The docs now say to add a rule for each.
Refs #306
* Mention repeated separators in the pypi rule note
Runs like "__" aren't collapsed either, so a__b is stored as a--b.
Refs #306
