Common::Client doesn't populate the prereqs info.

_populate_prereqs now doesn't do much things, only put a "skeleton" in place.
_populate_installedmodules only fill in the versions of toolchain modules listed there... probably because _populate_prereqs don't do anything yet.

The expected result is that _populate_prereqs actually populates what pre-reqs the dist asks for and that _populate_installedmodules looks for the version of them.
