no-ignored-test-files caches results too aggressively.

[jamestalmage]

It doesn't appear to pickup changes to package.json, only if I change the file under test.

It should abandon the cache if package.json has changed. (Or at least if the ava config portion has changed).

As an aside. Any linter rule that relies on caching should probably say so at the very top of the documentation.

NOTE: This rule relies on a cache to improve performance. Try `rm node_modules/.cache` if you think this might be affecting you.

[novemberborn]

Since the matching logic is executed in AVA code, we should be able to return a cache key that maybe the rule implementation here can use.