Add ability to clear cache

[josephdpurcell]

We have to use cache because the package has to be extracted (unless we can figure out how to stream read with pacote). So we can't have a flag to bypass cache.

But, we could add a command to clear cache.

Why? Because weird things happen. I just ran into a case where a package got unpacked without a chnagelog. I cleared cache and did it again and then it had the changelog, for that same version! It seems like a logical impossibility, but here we are and I'm sure it won't be the last time.