Use single "pull RSS feed method"

[shenanigansd]

Right now there is a function to get the package updates RSS feed, but it just returns the output of xmltodict with the wrapping headers removed. It will need to be parsed into a dataclass like the new packages feed.

[shenanigansd]

Actually, the feeds are identical except for one field, so lets just have one function that takes the feed URL and does the parsing.