Add perl modules to depends

Usage of ${perl:Depends} as documented at
http://www.debian.org/doc/packaging-manuals/perl-policy/ch-module_packages.html#s-module_deps
is misleading: it's *not* about perl module dependencies, but
instead:

| The program will look at Perl scripts and modules in your package, and will use this
| information to generate a dependency on perl or perlapi.

So we've to hardcode the list of Debian packages we
require for runtime.