Eclipse - DDT :: How to debug source files not located in 'src' folder

[Luigi via Digitalmars-d-debugger]

Hi everybody,
I am studying the cairod library and running some examples. I am 
using DDT and the example is a dub project. Moreover one of the 
source file of the program is outside 'src' folder where the main 
file is present.
My problem. Debugging the program it happens, going step by step, 
to pass to function defined into a package.d file, but DDT 
returns an info: "can't find a source  file at..." and I cannot 
follow anymore the rows under debug. I cannot even place a 
breakpoint. Even if at the location indicated by DDT the file 
exist and the source should be the correct one.
If I move that file within the 'src' directory I can still 
compile and I can follow the execution row by row also inside the 
moved file.
I am looking for a way to include among the source files, in a 
DUB project in DDT, a directory that is not the 'src' one 
foreseen for the project directory.
I've tried with gdb and it works. With gdb it was enough to 
include the directory includes the package.d file and I had no 
issue. I do not know how to let it works using DDT. I've tried 
several options but no one was actual. I did several searches on 
Internet with no good results.
Maybe the behavior is correct, I do not know. Anyone can explain 
to me the matter?
Thanks,
Luigi