Ensure that mod packages are installed before conf

Currently, there is no strict ordering that ensures
that the conf file is only created after a mods
packages are installed.

This means that it is possible for the .conf file to be
created before package installation.

In the case of libapache2-mod-wsgi (3.4-4ubuntu2.1.14.04.1),
this lead to a package install failure on Ubuntu Trusty

dpkg: error processing package libapache2-mod-wsgi (--configure):
subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
  libapache2-mod-wsgi
  Sub-process /usr/bin/dpkg returned an error code (1)

The order is ensured by this patch by assuming that the mod.conf
file is named: "${mod}.conf" (the same assumption made by the related
symlink) and placing a conditional dependency on all packages indicated
being required for the installation of the module.
what happens if either or both of those is undef?