packages cache (10 squashed)
Squashed commits:
[6856219] upgrader skin feedback for cached package
[69d16b2] fix upgrader bugs
[38ac884] fix cache key normalization bug
[930be25] clean cache with 1/50 prob
[fa6c9c0] update references to upgraders
[68a2939] plugin and theme custom upgraders using cache
[7e7b7a7] WP_CLI::get_cache()
[49cf57f] FileCache class
[835344d] add symfony/finder dep