Correctly pick up test-jar dependency sources

[keynmol]

fixes

test-jar dependencies not only have a different suffix for regular jars -tests.jar, but they also have a different one for test sources :D -test-sources.jar notice that it's test not tests.

They also need to have a different classifier (test-sources).

refactoring

resolveArtifact now takes sources: Boolean parameter instead of a wider classifier: Option[String] - we only ever use it to download sources, so we can tighten down the method contract.

I would also argue that resolveArtifact needs to return case class ResolvedArtifact(af: Artifact, sources: Boolean, file: File) so that we don't need to do this string manipulation with jar names, but it would involve larger refactoring than what I'm ready to commit to in this PR.

[tgodzik]

LGTM! Thanks for digging into this!