inflight installs, prevent peerDependency problems

Because of the cheesy backdoor mechanics peerDependencies use to get themselves installed, it's possible that more than one install to the same place can get queued up when peerDependencies aren't otherwise depended on. See the comment in the commit for details, and start getting used to explicitly depending on your peerDependencies, because that's the future.

The future is saner than now.