Support local file dependencies in Jar-in-Jar

[shartte]

This allows a much cleaner and simpler structure for including service-jars (i.e. core mods) based on including a secondary source set.

We use the filename as the artifact id, and its MD5 hash as the file version. This means two embedded local-file libraries of the same filename are compared using their content hash. If someone embeds a library via GAV and another embeds the same library as a local file, this fails. This is intended only for locally built source sets, and not for including local copies of Maven libraries.