[Future] Fix changes in the location of XML files

Fixes the problem that popped up after changes in LHCb!2851 (merged) and Detector!65 (merged)

CompactLoader        INFO +++ Processing compact file: /tmp/mimazure/tmpfEsYaT.xml                                                     
XercesC             FATAL +++ FATAL Error at file "", Line 0 Column: 0 Message:unable to open primary document entity '/gaussino_dev/lhcb-gaussino/Today/Detector_HEAD/compact/defaults.xml'
LHCb::Det::LbDD...  FATAL in sysInitialize(): standard std::exception is caught                                                        
LHCb::Det::LbDD...  ERROR Document::root: Invalid handle!
dd4hep: Error interpreting XML nodes of type <gdmlFile/>                                                                               
dd4hep: Error interpreting XML nodes of type <includes/>                                                                               
dd4hep: while parsing /tmp/mimazure/tmpfEsYaT.xml                                                                                      
dd4hep: with plugin:DD4hep_CompactLoader                     

@bcouturi I've used the trunk version for the Detector project. Not sure if that's correct.

cc: @gcorti