Slow initialization on each invocation

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When looking into #605, I noticed that it took a couple seconds before any output was given.

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After some investigation, I found that cargo package --list is taking about 100ms per package in a workspace. This happens even if we skip the package and even if the specific step doesn't care about cargo package --list.

I tried throwing rayon at it but it only offered minor gains. I'm assuming we are either saturating IO or dealing with lock contention. For project-specific lock files, I tried putting each invocation in a temp CARGO_TARGET_DIR but that didn't do anything. I did notice that the rayon output shows that its blocking on the package cache. I'm unsure if that is the main blocker or not. If It is, I'm a bit surprised and wonder if the scope of that lock is bigger than it should be but nothing stood out to be from browsing cargo's source code.