Fix mapSource invocation

[helospark]

PR for #44

What

• Moved out the mapSource to a central util to avoid code duplication
• Queried the method before calling it instead of relying on exceptions
• Added fail fast in case of the correct method is not found to ease later debugging efforts
• In JDT there is two overloaded methods with this name, but they are telescopic, and just pass null if the additional parameter are not filled. I followed the same principle to simplify the code.

Why

mapSource signature was changed in Eclipse Photon, however the invocation was still calling with the previous signature and did not fallback to the correct signature. This caused at least that: breakpoints cannot be set in decompiled classes, info is not shown for variables in decompiled class.

Please review, also let me know, if there are tests somewhere, which I have missed.