add a libapt-pkg recommends apt

add a libapt-pkg recommends apt

For many usecases like the acquire system libapt-pkg actually needs
tools and config found in the apt package. apt tends to be installed
everywhere libapt-pkg appears usually anyhow, but just in case to nudge
users (and tools) in the right direction.

Note that this isn't and shouldn't be a hard depends as there are
usecases working perfectly without 'apt' and as this is such an esoteric
problem incurring the costs arising from a Depends-Breaks-loop isn't
deemd as worth it.