clean up plugin service contract

[irainia]

Description
This issue is part of this discussion. There are two issues in the mechanism when we want to get upstream resource urn:

• we require the whole project spec and namespace spec
• the method name is not really clear, whether it's a resource name or resource urn GenerateDependencies

Acceptance Criteria

We need to refactor the plugin service to address the mentioned issues above.

• Change GenerateDependencies into GenerateUpstreamResourceURNs
• Ensure that when calling the method, only the required parameter is passed