Layout git modules on-disk as they are in XML.

As suggested by dfaure and at least one other person whose name escapes
me now, with all the modules available in git.kde.org it makes sense for
their on-disk layout to match the hierarchical organization given in
projects.kde.org, so kdesrc-build now does that.

If there's a big hub-bub about this I can make it optional, however I
think it should hopefully not be too hard to adjust to. A later commit
will add auto-migration of old source/build dirs because I love you all.