Automatically keep DABUILD_PACKAGES and abuild.conf's REPODEST in sync
Right now users have to be careful keeping DABUILD_PACKAGES and REPODEST in sync, otherwise packages won't end up in the bindmount that dabuild creates and as such won't be kept after the build. It'd be nice if dabuild were to automatically derive DABUILD_PACKAGES from REPODEST so users don't have to worry about that and it "just works".