implement apt.conf.d purging

implement apt.conf.d purging

this module already has parameters that let users purge sources.list.d
and preferences.d. It also has resources to create source files in
sources.list.d and "pins" in preferences.d and also has a resource to
create a configuration file in apt.conf.d. however, for some reason it
can't purge apt.conf.d

purging this directory can help users ensure that systems run exactly
with what puppet knows the system should be configured with.