cache: don't repack unnecessarily from tmp
cache: Always fetch root doc

Hitting CDN and fs caches is always way faster, even if it does mean
downloading more bytes on a cache miss.
test: test that cache loading doesn't mangle shasum
cache: Save shasum, write resolved/etc data to cache

Don't throw away that delicious metadata.
cache: treat missing versions as a 404

Regression caused by always fetching the root doc
npm-registry-client@0.4.7