support diffing final releases.

[mook-as]

• release-resolver: unify loading via releaseRefs
• release-resolver: use temporary files for URLs
• release-resolver: look up release info from final release tarballs
• cmd: skip normalizing release paths that look like URLs

Fixes #455.

I don't like this much yet; there's a bit with a hard :// check that smells a bit. It's also a lot of change in code I'm not all that familiar with.

:bug:Bug: this seems to accidentally move the final releases dir, which means we'll need to re-download things again.

[cfdreddbot]

:white_check_mark: Hey mook-as! The commit authors and yourself have already signed the CLA.