[cwconfig] Add a docstring on available_cubes method a renaming could be better at some point, but that's a start.

Backed out changeset fe995d56c949 available_cubes should not strip cubicweb_prefix but return actual package name for cube as package, since its output is also used to e.g. get ccplugin or site_cubicweb module name. Original aim of this set was to fix output of the "cubicweb-ctl list" command. This will be done by an alternate implementation in a later cset.