runctl: add npm ls-like package listing

Since it's supported by the runctl control channel, the same package
information will be available to the process manager/controller UI when
ready.

I'll add a test, but this is a light wrapper around strongloop/strong-npm-ls#1
Sorry, fixed. strong-npm-ls used to be lib/npm-ls.js, I guess I still had it hanging around after some rebasing (it was late at night).