Add a systemModules utility type

Storing system modules in a classpath is clumsy, as there should
only ever be one system modules, and it needs to store both a
directory to pass as the argument and a set of generated files
to use as dependencies.  Store them in a separate systemModules
type instead.

Test: m checkbuild
Change-Id: I020556c736bd5091865bcca51dc0fb9e4db6b45b