Find a way to remove empty stubs for mac compilation

[hidalgopl]

Currently to compile on mac os, I had to add _darwin.go files with Noop struct to avoid compile errors.
Related PR: #119

This probably means that our runtime abstraction is a bit leaky.
Ideally, we would like to have podman, itzo and mac runtimes as separate packages so we can add build tags (e.g. // +build !darwin) only to itzoruntime` package instead of adding it to multiple packages (as I did in #119 ).
Maybe it can be done by simply moving all itzo-specific packages (such as unit, net, etc.) to new itzoruntime package.

@ldx suggested that we can detect which runtimes are supported dynamically: #119 (comment)