ModuleInfo should have a notion of module visibility

[bartlomieju]

At this point we have 3 distinct types of modules that are executed in deno_core:

• internal modules - these are ext:core/mod.js and ext:core/ops - these modules are provided automatically by deno_core and have to be accessible only to other ext:* modules
• ext modules - these are all modules defined using extension! macro - these modules have to be accessible only by native code or other ext:* modules
• user code modules - these are all the modules executed after the runtime has bootstrapped - these modules mustn't have access to internal or ext modules. (Maybe there are situation where this is needed - in such case, there should be a method to expose them)