347319 Allow repository factories which load repositories by URNs

- Fix the response of the artifact/metadata repository factories in
  case that the location to be loaded is not a URL: the factories
  must return null or throw a ProvisionException with status
  REPOSITORY_NOT_FOUND in order to make sure that the manager
  continues to ask other factories. This allows to register a custom
  factory which for example looks up instances from a registry by
  uniform resource *identifiers* which are not uniform resource
  *locators*.
- Added a test for loading from a custom, registry based repository
  factory.

Bug: 347319 - [repository] SimpleArtifactRepositoryFactory prevents loading repositories from non-URL URIs
12 files changed