Spotlight can readily find .xip files downloaded on your system. It would be really nice if XcodesKit could discover these and use them as the install source instead of downloading the file from Apple's server.

A simple predicate of "kMDItemContentType = 'com.apple.xip-archive'" is sufficient to locate .xip files in all of Spotlight's indexed locations.

From there, .xip files can be matched to their remote equivalent by comparing their respective shasum values to the values listed in the XcodeReleases json data.