It seems like the common case for wanting to install a kernel module source package is to build the module and then install it on the same box. Since the *-source package doesn't really depend on *-utils, why not add a Recommends line? For example, ndiswrapper-source would Recommend ndiswrapper-util, while ndiswrapper-module-* would Depend on ndiswrapper-util still. (Similarly for alsa-*, and any other modules.)