module: only cache package main

Closes https://github.com/joyent/node/issues/6682

Locally, doing a simple server with hapi I observed 200kb of savings.  A package published to npm includes the README and LICENSE, so when we cache the entire package.json in this way we are adding a decent amount of memory to our procs that we don't need to.  Also, notice that `packageCache` was never exported or added to Module, so this isn't a breaking change.