performance: cache existsSync results in PackageCache.ownerOfFile

[raycohen]

Similar to #1608

A lot of time is being spent in PackageCache#ownerOfFile and a good chunk of that is from the calls to existsSync:

Before:

By caching, the amount of time in existsSync drops considerably in my large app, From ~12 seconds to under 500ms. The total time spent in ownerOfFile drops less, from 20 to 14 seconds.

After:

Even with the caching, this path is still so hot that there's a good deal of time in ownerOfFile and the new getCachedExistsSync methods that isn't accounted for by existsSync. There's probably more that can be done here.

[raycohen]

Looks like CI is having issues downloading volta

[NullVoxPopuli]

fix for that is here: #1594

[ef4]

Thanks, this looks good to me.

A nice-to-have in this area for someone to followup would be to refactor the existsCache and the rootCache into one thing, because I think an improved design would do both jobs. But that's not a blocker here.