L::Util: Require input files to exists

Require that the input file for read_dpkg_control (thus also
get_dsc_control) and get_deb_control to be present.

"Related" changes: Fix perldoc links for these subs and add a closes
in get_deb_control.

Signed-off-by: Niels Thykier <niels@thykier.net>