Support sourcepaths for lazy parsing

Right now there are two mechanisms to locate classes: by source(tree) or by classloader. If you use a sourcetree, all these files will be parsed. If you want only 1 file to be parsed including it's transitive JavaClasses, you'll have to parse the full source tree.
Let's add a mechanism where you can add sourcepaths, which won't be parsed by default but only when required (just like the classloader-mechanism).