Unify / Simplify module registration #220

With this change we now have 4 different type of modules (resolved in this order):

• host:<name> modules for host function calls
• user:<name> / <ns>:<name> for registedred modules
• <name> for native modules
• ./<path> / ../<path> for modules in the "filesystem"

I wonder if we could unify this or simplify it somehow. I think it's ok for this PR, but we should discuss how we want to evolve this.

Originally posted by @jprendes in #45 (review)