Bump model version to 6.0.0
Simplify interface Repository

findArtifact(): Remove boolean flag that controlled whether versioned
or versionless artifacts should be resolved.  Now it's totally up to
particular implementation of repository.
Prepare for merging Repository with RepositoryType
Change getArtifactPath() return type from File to Path
Fully remove RepositoryType interface
Rename SingletonRepository to RootedRepository
Add configure() method to Repository
Add initial partial implementation of AggregatorRepository
Implement RepositoryConfigurator component
Use RepositoryConfigurator to configure compound repositories
Rename AggregatorRepository to CompoundRepository
Remove deprecated class DefaultAggregatorRepository
Allow each repository to return multiple artifact paths
Remove deprecated class LocalResolver
Javadoc improvements
[test] Assert that repositories can be looked up successfully