This acts much the same ways as the npm help cache command line
The callback is called with the package.json data of the thing that is
eventually added to or read from the cache.
The top level npm.commands.cache(...) functionality is a public
interface, and like all commands on the npm.commands object, it will
match the command line behavior exactly.
However, the cache folder structure and the cache helper functions are
considered internal API surface, and as such, may change in future
releases of npm, potentially without warning or significant version
Use at your own risk.