OK, fixed this by adding a new module `easybuild.tools.module_naming_scheme.utilities`, which can also house the utility functions for making the implementation of custom module naming schemes easier (see #687).

I didn't consider simply adding a new module earlier to be honest, because I feel we have a lot of them already, and adding more only complicates things later when they also importing more modules from all over the place (which may happen to this one too in the light of #687)...