Finally implement compound OptDep group augmentation

Finally implement compound OptDep group augmentation

A group can specify not only what it requires (with includes), but can also
request a specific set of modules IFF another group is required within the
same call. This allows to do e.g. IC::DT+RDBMS dependencies in a sane manner.

Delete test_dt* groups from our optdeps (they were never documented, and a
cpangrep does not show any use) - instead we replace them with an elaborate
network of augments.