Streamline already-downloaded case

[adamwight]

The mix hex.docs fetch PACKAGE command is currently executed for every desired package, which is expensive and unnecessary. Make an API call to get the latest version and only fetch if a newer version is available.

Proof of concept in https://github.com/adamwight/local_hexdocs/tree/streamline-existing

[JamesLavin]

I wanted to do something similar but hadn't yet discovered an API for doing so. If you have, I'd love to reduce the # of API calls we make and speed up the fetching process.