20% npm install speedup: cache extras by package.json string

[mikesherov]

TL:DR

~20% speed up in the average npm install run when combined with npm/hosted-git-info#24

Details

This package used to actually cache package.json lookups, until b746f65 which was an attempt to fix npm/npm#7074 Deleting the caching mechanism definitely helped fix a correctness issue, but threw the baby out with the bath water. This PR introduces a less aggressive cache by caching by the true cache key, which is the string contents of a package.json file. Should be 100% safe to cache results based on this... I believe.

This lets us speed up builds quite a bit!

Before:

added 1764 packages in 51.943s
added 1764 packages in 51.135s
added 1764 packages in 50.688s
added 1764 packages in 52.048s
added 1764 packages in 52.156s
added 1764 packages in 54.817s

After

added 1764 packages in 42.428s
added 1764 packages in 45.116s
added 1764 packages in 42.085s

Let me know if you have any questions

[iarna]

This was merged into v2.0.7 as 839b56e. Thank you so much! =)