improve .meta contents

[devcurmudgeon]

as discussed on irc, .meta file could usefully include

• cache-key
• all factors that went into creating the the cache-key

this would require updating artifact-version too

[devcurmudgeon]

adding all of the cache-key factors will add approx 4000bytes to each .meta file, or up to 500kb for a system. i don't think it's worth it.