Let pip use a local download cache

This noticeably speeds up the time it takes to reinstall dependencies.
If the `.` prefixed targets are supposed to be "plumbing commands", `.clean-cache` is probably one of those commands that people won't need to run specifically or know anything about. `clean-env` should be exposed so there is a way of rebuilding the virtualenv without clearing the download cache by using `clean-all` (which would make caching pointless).