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
Bike shed, feel free to ignore: This is the only part of this PR that made me think at all when I read it. Would `false` make more sense here as a no-response indicator? Alternatively, the next line could just return, which is another convention I see often in Node code.