Improve source root resolving for multi-module projects

In #73 @fwcd mentioned some ideas for a more flexible submodule loading in the debug adapter.

The suggested solution would include querying Maven/Gradle for code structures, and use that to find the source roots. Currently, after #73, the source roots are searched for with a hardcoded depth of 2 directories. With a new solution, we would not have to make many assumptions about the users project structures.

Another hacky solution might also be a parameter for the debug adapter with `maxSubmoduleDepth`, but this would be another hacky solution.