Decide to use an intermediary class for generic drupal version information

imp is an analogous of migrate + migrate_d2d.
for sources, it would be useful to sometimes have extra information, i.e. if a module is enabled.
that could be achieved using an intermediary class between SqlBase and the real source class(i.e three clases: DrupalSqlBase, D7SqlBase and D6SqlBase, so we make sure we cover the relevnat differences if any)

@chx: what do you think?

btw this idea is also in migrate_d2d d7 somewhere IIRC.