Use cached npm info

[jablko]

• In generate-packages we know the npm info was cached in calculate-versions, so we don't consult the origin (npm registry) --- except in this one case, so fix that case (replace fetchAndCacheNpmInfo() with getNpmInfoFromCache()).
• Add some static checks that we're not going to the origin, except in calculate-versions.
• Don't bother writing the cache if there's no way we could have gone to the origin (correct but inconsequential optimization).

[jablko]

I've rebased this on #456 and replaced UncachedNpmInfoClient/CachedNpmInfoClient with pacote, which the npm cli uses internally for fetching metadata from the registry.

The advantages over UncachedNpmInfoClient/CachedNpmInfoClient are caching and concurrency control:

• Instead of hard coding whether the cache is fresh or stale (getNpmInfoFromCache() vs. fetchAndCacheNpmInfo()), using pacote.manifest() either way will use the cache if it's fresh and revalidate if not.
• There's one case where we already check whether the cached typesPublisherContentHash is up to date, which I retained.
• pacote handles concurrency internally via maxSockets, so wrapping it in nAtATime() is no longer needed.