Added some more methods to ArtifactManager that look to be needed based on...

Added some more methods to ArtifactManager that look to be needed based on grepping plugin sources for usages of now-deprecated getArtifactsDir.
Most such plugins just need to store or load individual artifacts, and could easily be made to accept an InputStream, and this is easy to support in various implementations, so do that.
A few really expect to get a java.io.File directory that they can play with, but these are the exceptions.
Such plugins will just not work when used on builds with nondefault storage, unless they are rewritten.