Genericize ref counting caches; fix extended config cache bug

[andrewbranch]

I plan to add another one of these shared caches for parsed package.json files for the auto-imports work, so I went ahead and made the implementation generic, which required some fiddling with config file parsing to not pass a closure on every cache lookup so we don’t also pass a closure on every source file lookup. I also noticed a bug with the extended config caching, which is covered in a new test and fixed.