Make ArtifactResult remember which Repository it came from for ceylon/ceylon.language#488: Ceylon embedded runner

So we can check if one came from a FlatRepository
Added FlatRepository for ceylon/ceylon.language#488: Ceylon embedded runner

This adds the "flat:" URL to map to a folder containing jars with no folder tree.
RepoUsingTool: allow getting the Appendable out for ceylon/ceylon.language#488: Ceylon embedded runner
AbstractCeylonArtifactResult: make resolving overridable on subclasses for ceylon/ceylon.language#488: Ceylon embedded runner
ModulesDependencyResolver: make the module.xml/properties file building accessible from outside for ceylon/ceylon.language#488: Ceylon embedded runner
OSGiDependencyResolver: do not prevent other resolver from trying to resolve, for ceylon/ceylon.language#488: Ceylon embedded runner

for cases where we have a MANIFEST but no OSGi declarations