polywrap-core package can be converted to namespace package with sub-packages

Refactored polywrap core may look like following:
- invoke
  - invoker
  - invocable
  - invoker-client, etc
- wrap
  - WRAP module
  - WRAP package
  - etc
- uri-resolution
  - Uri-Resolver
  - UriResolutionContext
  - UriResolutionStep
  - UriPackageOrWrapper, etc.
- client-core
  - client
  - client-config, etc  
