rethink scala and jvm specific concepts vs. general ones

[cvogt]

currently dependencies are THE dependencies for a build. and sources are THE sources, where sourceFileFilter decides what is part of THE sources. And exportedClasspath is only exporting a classpath.

A build can produce different things though, not just a classpath, and sources can be different things, not just .scala files, also .scalatex files etc.

so either we come up with a concepts of many types of dependencies, many types of produced artifacts, many types of input sources and a way to have them all co-exist in one build. Or we force only one type per build and allow many builds on the same directories, depending on each other. The latter might be clearer, but can we do it without overhead?

related to https://github.com/cvogt/cbt/issues/239 https://github.com/cvogt/cbt/issues/438 https://github.com/cvogt/cbt/issues/437