Test coverage for IDE generation with missing source/javadoc artifacts
Replace several uses of ArtifactIdentifier with ModuleVersionArtifactMetaData
Unpack the moduleSource when delegating from caching repository to external resource resolver
Don't assume existence of source/javadoc artifacts with maven scheme

- We now check if artifacts exist and only return those that are detected
- Artifact resolution api no longer returns 'failure' results for missing
  source and javadoc artifacts on maven modules