Cache Garbage Collection

[mischnic]

1. Store all cache keys related to Transformation and packaging in the request graph
2. Walk the request graph/bundleInfo list to figure out all "used" cache entries
3. Traverse all existing cache entries and remove all unused entries

The big question: how often should it run. Right now it:

• runs after one-shot build if the last GC was >=24 hours ago
• runs if watch mode if the last build was >=5 minutes ago, and the last GC ran >=4 hours ago. (If the GC takes too long and a build is requested in the meantime, it should abort the GC.)
• Manually (forcibly) via Ctrl-G in watch mode

Any thoughts on:

• this approach in general (build vs watch mode, waiting for idle)?
• the specific durations?

Problems this causes:

• with parcel serve && parcel build, the prod build could remove all cache entries from dev mode (or if there are multiple Parcel invocations with differing entrypoints)