archive: committable offline dependency archive

[zkat]

This adds two npm commands: npm archive and npm unarchive, which are integrated with libprecious to provide a full-fledged committable dependency archive experience.

[zkat]

I'm going to close this for the time being -- we want to redesign this to be way better, and this thing as-is conflicts with future plans we have for how caching and installation work. We'll revisit this idea in the future, but it's not very likely to look exactly like this. I'll leave the code up for the curious and those who want to manually patch their clients or just for reference. I'm also going to stop including this command in the canary, so npmc@6.4.0-canary.0 will be the last canary version with npm archive built in.