Fix handling "libs" on Windows for BuildMode.allAtOnce.
Fix handling "libs" on Windows for BuildMode.allAtOnce.

Libs were mapped to sourceFiles and then ignored. Now they get correctly mapped to dflags.

p0nce/dub,D-Programming-Language/dub,Geod24/dub,rjframe/dub,jeanbaptistelab/dub,Flamaros/dub,jelmansouri/dub,grogancolin/dub,gedaiu/dub