Integrate cacache@6

[zkat]

cacache 6 involves some big changes! Most notably, changing a bunch of stuff to be Promise-based, a new on-disk format, and moving all the memoization code out of pacote and back into cacache itself.

As part of this integration, pacote itself should be updated to use Promise, the lib/cache code should be torn out, and cacache should be used directly.

This is gonna have to start before cacache@6 itself is tagged because I really wanna know the API changes are good and we don't need to move anything else in there.