Using `SOURCE_*_TAR_GZ` changes nothing, it's just a constant that does the same as e.g. `"%s-%s.tar.gz" % (name, version)`, but without using Python syntax string templatng.

By using the `system` module class, suddenly `hwloc` will no longer be (symlinked) in the `base` dir for module files, but in `system`, so we're changing behavior here...