Dpkg::Fields: Change set_field_importance() into a method of Dpkg::Fields::Object

Dpkg::Fields: Change set_field_importance() into a method of Dpkg::Fields::Object

* scripts/Dpkg/Fields.pm: Drop public function
set_fields_importance() in favor of a method on
Dpkg::Fields::Object. Integrate public function
sort_field_by_importance() in the output() method where it was
used.
* scripts/Dpkg/Changelog.pm, scripts/dpkg-genchanges.pl,
scripts/dpkg-gencontrol.pl, scripts/dpkg-source.pl: Updated to use
the set_field_importance() method instead of the removed function.
* scripts/Dpkg/Control.pm: Update pod documentation to refer to
Dpkg::Fields::Object instead of the invalid Dpkg::Cdata::Object.