Properly implement 'Large refactoring...'.

In a previous commit I made a "large refactoring" that involved using
mixin classes to decide at runtime what build system or scm handler to
use for a given module.

In retrospect, using what is essentially a static class member to handle
a per-object decision was not my most impressive moment ever... this is
easily fixed by having specific scm() and buildSystem() members for each
Module instance, but does involve yet-more modification of code in lots
of places. :-/