Respect cache-control header for simple responses (#13955)

This reverts a change from 2018 where simple pages would always be
revalidated on every request. This ensured that all changes would be
visible immediately (while minimizing the number of bytes sent on the
wire), fixing one scenario while penalizing everyone else. In
particular, this fix is expensive whenever pip is run multiple
times in short secession (like tox workflows), especially on high
latency connections.

For people who need pip to be able to find newly published packages
immediately, they can use the newly added --refresh-package option,
which was inspired by uv (which doesn't force revalidate unlike pip).