The hashlink github contains several haxelibs inside it, including the "hashlink" repo itself.
If you mount this, and try to output a project via -hl somefile.c you'll get an error
Error: Error parsing haxelib.json for hashlink@dev: [file_contents,/home/spitko/work/bugrepo/haxe_modules/hashlink/haxelib.json]
Error: Build failed
Similarly, to actually mount libs inside this git project, you have to -cp and -D them individually. This part is mostly an acceptable compromise for a weird edge case, but the former issue is more problematic.